I think I just fixed my issue. The saved files were in a folder listed as 'StrandedInSpace-1.75-pc/'. The update created a new base folder listed as 'StrandedInSpace-1.81-pc/'. So I copied over my save files, loaded the game, and saw my saves. Yay. I then removed the old game file from Steam, and added the new one, and it's opening from my new protonGE 8.6 and my saves are there(I previously removed 8.4 and installed 8.6 when troubleshooting). Now to test if it works in Game Mode. Will inform of results.