I have a problem with finding a suitable mathematical journal (maybe not of the highest level) that accepts longer paper (say 100 pages). I know only two such journals: Memoirs of AMS and Dissert.Math.
But both journals now are in Q1 quartile, so are very selective, without any chances to be accepted there.
In fact, "Dissertationes Mathematicae" was created in 1951 (as "Rozprawy Matematyczne") with the purpose of publishing good Ph.D. Theses and indeed some time ago it was quite real to be accepted there. But since 2019 when they entered to the Q1 quartile this journal accepts only papers of exceptional quality (so far higher than an average Ph.D. Thesis).
So, what would be your suggestion concerning journals for longer papers (at the moment I would not like to consider splitting the long paper into smaller pieces). Thanks for the advise (I suspect that many mathematicians had such problems in mind, so the collective experience had to elaborate some helpful answer here).
Best Answer
Applied Categorical Structures https://www.springer.com/journal/10485/ seems to match the topic of your paper and fairly regularly publishes 70+ pages papers. It is a Q2 journal according to Scimago.