I dunno if all ADHD is exactly like yours, but I doubt it. Your version sounds pretty hellish, to put it lightly.
Good info though, thanks for deeming us worthy of your time (not sarcastically) and sharing your experience. Pretty eye opening for us neurotypicals.
BusyCal as a macOS client and baikal as a server.
Those two are reliable as can be. BusyCal is a great app with a responsive developer and can connect to virtually anything.
As to the topic of this post, despite the above, I’m trying to move away from CalDAV and CardDAV, the latter has poor support on most mobile platforms in the way I want to use it. I also am trying to reduce self hosted platforms I maintain, which is currently a high number.