As an update: I've been able to read St. Thomas's Summa Theologiae and Cajetan's commentary on it without too much difficulty even if slowly at times (mostly due to some new vocabulary). It has been rather heartening that the habit of reading Latin that I learned through reading Familia Romana is a real habit and so I really am able to (mostly) simply read these other texts. This is opposed to learning Latin only through the grammar method and so do a lot of translating (with great difficulty) and never really being able to read the text.
One question I have: Cajetan, an early 16th century author, often has "ly" in his texts, but I haven't found it in any dictionary and can't figure it out.
To give an example:
In titulo huius primi articuli, tres terminos nota. Primo
ly necessarium. Dupliciter enim sumitur V Metaphys.*:
simpliciter , et ad finem. Hic sumitur secundo modo. -
Secundo ly pltysicas * disciplinas.
Actually I've noticed it is often after numbers, but here are two cases when it is not:
Et quoniam talia sunt subiecta
qualia permittuntur a praedicatis, consequens est quod ly
sacra doctrina sumatur hic pro doctrina revelata ut est conclusionum.
Sed theologia nostra distinguitur, si ly nostra
demonstret huraanam naturam simpliciter, in theologiam
beatorum et viatorura.