The problem with Mass IX is twofold from my perspective: it's infrequent, and the range is tricky. It's OK at the 10:00 Mass on Sundays or holy days where it's a Saturday or public holiday. It's not so OK in the morning (hello, Rorate Mass) or evening. I'd go for X actually, if my priest was able to intone it, at the Rorate Mass.

However they sing the rest of Mass IX with no problems so long as we nail the transition from the first Kyrie to the second; this is also one Mass where I not only prefer alternation schola-congregation, I prefer the women to take the lead. That solves both the second Kyrie and the "suscipe deprecationem" in the Gloria. I'll say, it took years of listening to the Fontgombault Assumption CD before really getting it, as I rarely had the opportunity to sing Mass IX in the wild; Marian feasts at my old parish always meant polyphony for the Gloria…

Also, Patrick's being a little modest. I couldn't make the full sung/solemn Mass, but it was nice to have a sung Mass (reduced propers, but you could easily do full propers in theory) instead of just low Mass when I was in the area this spring. I also happen to agree re: the Kyriale in particular the ad libitum chants and some of the Masses for double feasts… there's a reason that not even Solesmes has them in the regular rotation (as far as I am aware).

The usual I, IV (less usual but it's the default for Holy Thursday in the old books, and is really lovely), VIII of course, IX, XI, XVII, and XVIII (and the special Kyrie pro defunctis) plus or minus one or two Masses make sense not only because you'll cover all of those days with Sundays, holy days, and Ash Wednesday or All Souls, funerals etc. but because with some practice, a congregation can come back to them even somewhat infrequently (IX in particular). I think that X, XII, and XIII are all wonderful options but you have to choose, for example, between a Gloria with a hard incipit (X, XII) and repeated ascending fifths for those singing (XIII); by its nature, a Kyrie with the same leap is easier if the congregation is habituated to parroting the previous strophe, which is normally identical.

Also, our schola has the fun task of shepherding a very strong congregation with Ite XI. "But Matthew, it's so easy, and you do it the most often." Well exactly, it's just that the syllabic division is unusual if you compare it to the others, and they don't listen to us, they follow the minister (priest or deacon) who may sing "I" as La-Si flat-La-Sol (what you'd expect from the other Ite chants) instead of La-Si flat, which is what is written.

Upon reflection, given that most Sundays are feasts of the II Class in the 1962 Missal, there is plenty of scope to use the Mass settings provided for those feasts.

"Congregations seldom seem to complain about straining upwards in their annual rendition of ‘Hark! The Herald Angels Sing’, which is done so often in G because of the ubiquitous Willcocks arrangement."

A. It's a very familiar tune, from saturation over time, and people retain a good pitch memory of it.
B. It's usually the recessional rather than the processional. People who sing will have more of a chance to be warmed up. At midday and evening, even more so; in the first half of the morning, much less so.
C. One can do something other than Willcocks. One could also play in F.

Of course you could sing something else..., most of the assignments in the Kyriale appear to be made up around 100 years ago, the 1903? Kyriale does not have the same assignments.

Yes... you can even mix and match movements from Mass to the other taking mode or mood into consideration.