JetBrains Rider seems to do a good job for exploring assemblies, but it's kinda spendy. I used it for the 30 day trial, and wish they had a community version like for their other IDEs.
On the topic of mono versions, I've taken to invoking mcs with -notstdlib, and then listing the specific system libraries I want used (RimWorld/RimWorldLinux_Data/Managed/System.dll et cetera). This seems to be the most stable way to get the right versions of things (3.5 for RimWorld, 4.something for MonoGame projects).
The full command is something like
mcs -nostdlib -target:library -out:../Assemblies/ModName.dll `find . | grep cs$` `for f in $Reference/*dll; do echo -r:$f; done;`
Where Reference points to the Managed folder (or wherever the appropriate mono system libraries are located).