I didn't answer because you didn't give sufficient information about:
a) why you wanted a Modula-2 compiler?
b) why you particularly wanted to install gnu modula-2?
c) why you had a problem installing gnu modula-2. Was it just the size that was a problem?
d) was there any particular feature of MacOS Big Sur that you need support for?
e) does it matter whether it is 32-bit or 64-bit?
f) were you looking for freeware or a commercially supported version?
The more you tell us about your requirements, and which are 'must haves' or 'nice to have' the more we can help you with exactly what you want or maybe some alternative possibilities.
P1 Modula-2 is a commercial Modula-2 compiler for MacOS that is still actively maintained:
If you want to investigate other Modula-2 compilers for other systems, here's a comprehensive list: