Lots to respond to here. But as for your last paragraph, the big thing that's "missing" in Monastic Compline is the Nunc dimittis.

We are talking of the 1920 Pius X breviary which preceded the current LOTH. The liturgy went through a number of revisions between 1955 and 1969:

Rubrics 1960
Reductions to Simpler Form 1955 (Cum Nostra Hac Aetate)

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De Anno et Eius Partibus (English translation from 1967 Dominican Breviary)

Rubricarum Instructum

General Rubrics

Rubrics of Breviarium Romanum

General Rubrics of the Roman Missal

Tables of Feasts

1960 Roman Calendar

Variationes in Breviario et Missali Romano

Variationes in Martyrologio Romano

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26 September 1964 decree Inter Oecumenici

4 May 1967 decree Tres Abhinc Anno
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As far as I can see the standard assignment of psalms did not change at any time :-
https://www.divinumofficium.com/www/horas/Help/rubrics.html
and for Compline this assignment always held (outside the Triduum), different every day of the week;
and as far as I can see the first mention of adopting Sunday psalms every day is in 1967, permitting it where Compline is recited with a congregation (no mention of private recitation)

I am of course open to correction.

Correct, and it was also, in the Roman rite, pss. 4, 30 (just part of it, the part essential to Compline), 90, and 133.

The “loss” of the Nunc Dimittis has kept me from the Benedictine office and it may be a stupid reason to reject a vocation, but hey, I think of how unsearchable His ways are all the time.

I have personally thought that the shift from ps. 118 at the Little Hours was a mistake, and the rise of liberalism even inside the seminaries as detailed, for example, by Mgr Tissier de Mallerais in the biography of Mgr Lefebvre was perhaps bound to happen anyway, but it’s a little on the nose. (A good way to pray that psalm is to say “Jesus” mentally when you get to words like “lex”, “mandatum”, and “justitia”.)

As far as reformed versus traditional Roman Compline, regardless of the psalter:

• the antiphons never changed except by those of the daily psalms after 1911 or in Paschal Tide (and there, the Sunday antiphon is repeated daily even after 1911) with one exception, which is particularly helpful as the Nunc Dimittis is therefore stable.

The exception is on Holy Saturday; the antiphon Vespere autem sabbati of Vespers is repeated, and the canticle actually loses its antiphon for the paschal octave, but the tone is a variation of familiar plagal tones (sort of mode 2, sort of mode 8) and is also used for the psalmody of Compline on Holy Saturday (and ad libitum in Paschal Time at Benediction). In any case it’s an easy office to learn to sing.

• I personally find making the hours identical in structure to be less conducive to Compline as night prayer and Lauds and Vespers as the two principle hours (this is what the council explicitly envisioned!). Before the reform, the hymn of these hours is in the middle, ish.

• On a similar note, the old office begins with the blessing of the lector and “Noctem quiétam et finem perféctum concédat nobis Dóminus omnípotens.”. This is moved to the blessing of the end of the hour in the Liturgia Horarum, and the 1 Peter 5 reading is gone. The confession is done more like the Penitential Act of the NO, and the transition to the rest of the office is abrupt (iBreviary doesn’t even have that in private recitation; this part is obligatory in private if you use the preconciliar office in any way).

• the chapter from Jeremiah is replaced by a variable reading.

• the collect Visita quaesumus now changes (or there are options; iBreviary is not a reliable source, but in any case, it’s enough to show how unstable and dependent on choice it’s become).

• the hour loses its unique ending (Benedicamus Domino, followed by a blessing). The Marian antiphons lose the versicles and collect followed by the versicle and response “Divinum auxilium maneat semper nobiscum. Amen.” So the office just sort of ends, and this even in the Triduum, where, following the traditional rite, the antiphon is paused after Compline of Spy Wednesday (including after Lauds or the last hour sung in choir and then Vespers if separated from Compline before 1955) and resumed after Compline of Holy Saturday.

Wow, that 1907 Solesmes book is interesting. There a a lot of small differences in the chant tones, more significant I think than the textual differences.

Yes, it was the Nunc Dimittis that was missing, I just couldn't remember through my brain fog.

The “loss” of the Nunc Dimittis has kept me from the Benedictine office and it may be a stupid reason to reject a vocation, but hey, I think of how unsearchable His ways are all the time.

It's funny you should say that, because the very same thing made me much less inclined to joining the Benedictines as well. I thought "Sunday psalms every day isn't that bad, but losing the In Manus Tuas and Nunc Dimittis would really hurt".

Looking at the 1934 Antiphonale Monasticum, it seems the Nunc was only included on Holy Thursday and Holy Saturday at Compline, and on All Souls.

Those were days where they used the Roman office in whole or in part.

Compline and Little Hours of the dead were also a bad reform of Pius XII…

The Dominican Compline had 2-3 psalms every day. During Lent special responsories and special super psalm and/or Nunc Dimittis antiphons for special feasts. Coming from the OSB tradition it was quite a change.
The traditional OSB Compline I know had the Nunc Dimittis but not an antiphon before it.
All by way of saying that the Divine Office has a lot more variety than most of us are really aware of.

Does anyone know if there would happen to exist any rubrical loophole or fine print that would allow FSSP priests to satisfy their obligation of saying the office by saying Sunday Compline instead of the weekday psalms? If we were allowed to do Sunday Compline all the time, our pastor might be willing to have the whole parish chant Compline more often. But he's not going to chant something that doesn't even count for his office obligations. He refused to anticipate Tenebrae with us since he would just have to say it again in the morning anyway.

When we have had guest priests over for dinner, I point out that we always sing Compline as part of our night Prayers. I have offered to modify our practice to follow 1962, but every priest seems happy to follow our rubrics (usually D.A.) and Calendar (c. 1935).

@OMagnumMysterium
I have not seen the other book, but it looks similar to the old Baronius Press book.

For Sunday Compline we usually sing from memory, including the psalms, and the children seem to know the melodies and the doxologies of the Hymn from memory.

We have a set of these, https://introibo.net/komplet.php
I use "Vesperae et Completorium de Dominica pro Diversis Temporibus Anni, cum Cantu Gregoriano, No. 902" or the 1907 Compline book (when we use the older tones of the Hymns) or my 1949 Antiphonal. We also have a couple of copies of the Angelus Press book.

My older children use the Liber Usualis for anything they have not memorised.