VS Code (i know it's still MS but I do C# .NET work and rider is too expensive, I don't want a subscription for an IDE)
VSCodium is a thing too if you want to un-Microsoft even further.
I use it for C# development on Linux and it works well.
getting a password manager
Bitwarden and Keepass are usually the go tos, depending on your use case.
then a new browser
Firefox or if you want to decouple from Mozilla as well, Librewolf works pretty well.
potentially a Google pay replacement
I'm not aware of any open Google Pay replacements other than taking a card with you.
As soon as you get rid of Google on your phone, you get rid of Google Pay.
Is there something missing in OmniSharp that prevents you from using VSCodium?
I do most of my C# development with the OmniSharp plugin in VSCodium on Linux.