The March 2023 update for Parkitect has broken my campaign menu (can't go back to main menu or launch any of the levels) and killed all my progress. I still have the save files from each of the finished campaign levels, though. And I'm not the only one. My SO has the same issue on a completely separate GOG account and PC. Any ideas how to fix?

How did you update the game? You can press escape to get back to the main menu on the campaign world.
Now i didn't lose any progress as i only completed 2 levels and i do have all dlc installed.
Did you use offline installers or gog galaxy to update?
Also i do know that the save files are in the documents on pc win 10.
I reinstalled the whole game when the new version came out since i am a little confused with the dlc.
Think you have to install 1.8p version of main game and then the 1.8p dlc versions.
Don't think you can just install the update when the previous versions of dlc are on it
I can run the levels though since i didn't get far.
Also used offline installers.

Anyway a side question how much do you like the game?

GOG automatically updated the game. Pressing escape from the campaign menu doesn't work. I also can't click on any of the levels. The ability for multiplayer/co-op is really the clincher there.

I like the game pretty well. Only bummer that I discovered after playing for a bit is that apparently the majority of the mods are only available through Steam workshop. So I partially regret buying it on GOG.

Don't know what happened to your game, don't know if gog has a verify repair button or you could contact use the gog support for it. Maybe a new update can fix it when it comes out.
The mods it depends on what mods you are interested in.
Some recent parkitect review on gog says the mods aren't really anything new and could get them elsewhere online, but not all.

Also did you try any of these:
https://support.gog.com/hc/en-us/articles/212454049?product=gog

Try running as admin

The mouse and keyboard work everywhere in the game except the campaign menu. So I don't think that page is the issue.

I think I had the same issue, although I could click on credits and then press escape to return to the main menu. I noticed that changing my current graphics to either windowed, borderless window or fullscreen would allow me to click on things in the campaign menu. But progress through the campaign would not be saved between sessions, and the clouds would not disappear after a certain point.

My next idea was to backup and then delete my configs/saves at %USERPROFILE%\Documents\Parkitect, and disable cloud saves temporarily. Although I ended recreating %USERPROFILE%\Documents\Parkitect\Saves\Savegames\, and filled it with just the old .park files. This fixed the campaign menu and progression issues, but I lost my achievement progress.

On closer inspection, my backup contained a campaign folder but the new working version was a file. So you could try just deleting that folder instead, %USERPROFILE%\Documents\Parkitect\campaign\.