Linux user here. I don’t know of an open desktop calendar app that supports the protocol I need (CalDAV) without being one or more of:
Bloated
Too simplistic to be useful
Too annoying to use (poor UI)
The best compromise I’ve found so far is Thunderbird. It is bloated, but less so than any Electron app I’ve used. I find the UI annoying, but tolerable for lack of a better option. I’m thankful for an open, cross-platform tool that gets the job done, but I wish I had one that was lightweight and pleasant to use.
It would be nice to see some new work in this area. It’s a similar situation with email apps.