We have the illusion that the Tridentine Rite, in fact all Roman Church teaching, was followed fully to the letter of the law. The reality could not be further from the truth. There have been localized, even micro levels, variations that have made the Roman rules achievable and meaningful to the faithful. The US church was fed a fantasy by well-meaning proponents of the praxis of Catholic Catholicism. From the long view of church history, the Second Vatican Council in all of its documents codified what was always the prevailing practice of the Church. Inculturation has always been normative if often unrecognized. Many historians would say, Church has never been as powerfully centralized as it at this time in history. The Roman Rite has always been a very big tent in which the faithful could find a path that meets their localized or particular tastes

Missa de Angelis and Credo III are among the most singable options in the chant options. They are repetitive, sequentially developed, sound as in a major key, with it's of repeated scale-based motifs. No wonder they are still "in the ear" of so many.

I grew up in a country parish in Washington State. We had a small choir, mostly women, which sang traditional hymns from the then standard Catholic hymnals, St. Basil, etc.; what we now call the four-hymn sandwich was alive and well there in the forties and fifties. In my recollection, the congregation never sang anything. Other churches, however, had more ambitious music programs.

My parish (Polish--USA, New England Mill Town) seems, based on a recording released, some anecdotes, and archival evidence, to have typically done a kind of Singmesse, which was common, since the Church in Poland had an indult to replace the Mass Propers with vernacular hymns which was granted during the persecutions during the Partitions (Poland conquered and divided between Austria, Prussia, and Russia---religious persecution felt most strongly, of course, in the Protestant and Orthodox-ruled sectors), and came over to the USA by immigrants.

The (sung) Propers were replaced with devotional hymns or Mass hymns (the priest, of course, recited the text of the Propers normally), but the Ordinary was always sung in full, usually in a 19th century setting, but Haydn and Mozart were sung, too. However, my parish also has some well-worn books of Propers, most in some kind of Falsobordone setting, though some chant books, too, so there were times when hymns did not replace Propers, or not all of the Propers. And the Requiem Mass seems to have only been done in Plainchant, as witnessed by the extremely worn-out 19th century book of pre-Solesmes plainchant for various classes of Requiems, and including the Polish devotions for All Souls' Day--plus a MS addendum in modern notation of the In Paradisum, with rudimentary harmonies: they were definitely singing mensuarally. And from how I remember some of the "Old-Timers" singing plainchant in my childhood in the 1990s when our now pastor-emeritus began implementing RotR before it had a name, they must have sung their chant mensurally until the 1970s when a new pastor came in and swept everything away with a full-on implementation of the Novus Ordo, including adding guitar Masses, etc.. (Of the chant books in the archive, the Solesmes books seem almost unused, while pre-restoration books have been written in, torn, taped, patched, re-bound, etc.)

Incidentally, the pastor at the time of the Council and its immediate aftermath (Fr. Joseph Szczepaniak) and his immediate predecessor (Msgr. Col. A.A. Skoniecki)--they swapped parishes in the 50s--were the last two holdouts in the Diocese against Mass Versus Populum.

From the 1936 (published 1940) revision of the Westminster Hymnal, in the Preface

"... the norm of a Catholic hymn is the ancient Office hymn ..."

"... some of Neale's hymns may be alien to those who have been raised in the atmosphere of the homely Catholic services of the last 50 years, with their loud and draughty singing"

And in the editors' note

"some of ... Latin hymns ... might well be sung by the congregation at Mass in place of the customary motet"

That last is the only indication that any hymn could be considered for Mass. Westminster of course has very few country parishes.
I think I remember looking at a different edition which said firmly - these hymns are not for use at Mass.

The Singmesse is still quite common for traditional Masses (SSPX, FSSP, Institute, diocesan, etc.) in the German-speaking world. If it's the full "German High Mass," the congregation and choir (if there is one) doesn't sing anything in Latin except the responses. Everything else is a either paraphrase - not a literal translation - of the Latin ordinary or else a strophic hymn in some way suited to the part of the Mass. The German-Latin hybrid form is also common, where the ordinary is sung in Latin and German hymns - or perhaps instrumental music or motets - are substituted for the proper. The 1955 norms from the Holy Office approve the reverse, namely, the proper in Latin and the ordinary in German paraphrase. That is referred to as "Sung Popular Mass" (Volkschoralamt), but I suspect the practice died out.

A Mass with a familiar ordinary, familiar hymns, and a competent organist requires considerably less preparation than the full Gregorian proper and, in my opinion, is more musically and spiritually satisfying than psalm-tone chants or other substitutes for the full proper. Such Masses did and do exist! I would add that they were not only a country church phenomenon. They would be common even in cathedrals with great musical traditions for Masses other than the principal parochial Masses. You might find choir, orchestra, and men's schola for one Solemn High Mass on Sundays and holy days and German hymns and organ for the other Sunday Masses and practically all public Masses throughout the week. That model has been carried over into novus ordo practice in parts of Austria and Germany.

How prevalent were full Gregorian propers before the changes? It's hard to say. I suspect that Sung Masses in smaller churches were uncommon. It's all before my time, but some older Catholics have told me they remember Latin ordinaries, motets, and hymns in the old days, but no "chanting." If the propers were sung poorly, or to psalm tones, or recto tone, maybe they weren't very memorable, but perhaps the recollections are accurate and they were omitted by the choir and simply read by the priest as always. I've also been told that Low Masses were the norm for weddings and funerals. Today the traditional communities always want to impress the novus ordo visitors with their nuptial and Requiem High Masses, but back then, there were none to impress! With all of the recordings and free resources now available, the full Gregorian propers are probably more accessible than ever, even in parishes with the most modest resources.

pretty good idea of how the Mass was actually celebrated

Maybe not.
There was a lot of self-selecting there. The people who cared enough about music to read and write to the Caecilia were writing about their best Masses, not their typical Masses. At most, you can say that such reports represented the exemplary Masses of the age. The dozens of music directors who didn't belong to the Society (to every one who did) aren't heard from. Consider: when we have a thread like "[liturgical holy day] at your parish", are those Masses at all typical of what goes on in the US? If they are, there's a lot of baseless grousing here, because most of us are doing pretty well.

writing about their best Masses

Perhaps.

I would point out that there are some great question and answers discussions answered by Dom Gregory Hugle, O.S.B., that are very interesting to read. And, there are many articles covering the music of the "Low Mass" in several of the magazines. I just think the Cecilia magazine is an often overlooked resource. There is also the "Catholic Choirmaster" magazine but I have never looked through any of these.

Papa Pio Nuh-Uh: La tradizione è ciò che mi piace.

For Ruthenian Byzantine Catholic, check out this recording from 1966. Stick with it long enough for the Slavonic to kick in, after the first litany.

The resources we have now far surpass any of the past in both quality and accessibility

True in the sense I'm sure you meant it, but human resources (as much as I dislike the term) are often spread very thin in TLM communities. Being deprived of the traditional Mass in our territorial parishes doesn't help matters. In the US, I would guess it's not uncommon for people to drive through a dozen or more parishes to get to a traditional Mass. And unlike the "reverent" novus ordos, the TLM is often in a bad part of town. Anyone who lives within walking distance of a TLM nowadays should certainly count his blessings.