I use nextcloud tasks and deck for keeping track of things. Tasks is great for checklists, calendar integration, reminders, etc., I use it for simple reminders like trash day, and things I don’t want to bother with making a calendar event for. You can have it make a calendar event, but I don’t. Deck is a kanban style thing with calendar integration as well, it’s a bit more job-task oriented so I use it to help me juggle tasks when I have a lot of projects that need keeping up with. Setting up nextcloud is way overkill for just those features, though. If you’re not looking for the other features of nextcloud, I don’t think it’d be worth it.

It is pretty easy to set up if you go the nextcloudpi route on Debian, you don’t need a raspberry pi. Any computer running Debian can do it, but you may need to install a few dependencies if you’re on minimal Debian that aren’t listed anywhere, at least last time I set it up they weren’t. But again, if you aren’t looking to use the rest of nextcloud, I wouldn’t go that route.