@Quasar Thanks a lot for your help! I succeeded compiling it by adding all the .dll files manually in .csproj as @phuoc.tran suggested here, but would love to try your solution too. I just cannot fully compile MonoGame right now due to a non-code-related issue (or can I?).
I have another question—if I understood correctly, does this mean that High Sierra (10,13) and Catalina (10.15) return different values on new Uri() command? Do you happen to know what makes this difference? Assumption/speculation is welcome. Thanks again!