Therapieauslass

04:14 May 9, 2020 

(1) I don't think the redundancy of the word "therapy" is particularly objectionable. (2) As to "post the therapy", Google hits are reasonably abundant. I defer to your judgment -- I am infected by having read many hundreds of physician notes (in EN), in which murdering of idiom is notorious.
INCIDENTALLY (important): I accidentally left out the Alternative translation in which I used "post the therapy".
Namely:
Following a course of therapy of more than 20 months, a relapse occurred 5 months post the therapy.
The main point I tried to make is that the prefix (or etc.) "post" is very common and accepted in both informal and formal medical writing, and is rare in non-medical writing (e.g. in engineering writing).