More of a collection of suggestions:
No matter the platform, which proprietary software have you replaced with open source ones that function the same or better? I'll start in the comments.

I have migrated from several clpud stprage services to Nextcloud. First of all, it allows me to keep my data for myself, and also functions a lot better (speed compared to Google fucking Drive). I also moved my contact and calendar sync from Google to the very same Nextcloud.

Operating system, various audio/video/image codecs. Compiler and IDE, browser, media players, games. Virtually everything.

Although I obviously didn't replace all games yet because theres no open source witcher 3.

Might not be proprietary but I've replaced Unity with Godot Engine some time ago.
Great decision!

Removed Windows that came with laptop and installed Linux 😅

Replaced Photoshop with Krita