For file syncing, I would highly recommend owncloud over next cloud for self setup. I’ve tried next in a few instances and they are much more feature rich but not nearly as reliable in my experience and requires more maintenance. Owncloud is simple maybe even too simple for you, but sync features for me work out of the box. For computers just download the app. Point it to your server and you’re good. For mobile there’s an app for it though it is more limited than the desktop app/next cloud. You can look at videos too and make your own decision but that’s my 2 cents. I currently run it off proxmox.
For docs, I’ve also run into issues with Joplin and haven’t moved from onenote yet. I tried orgmode but again that requires another technology stack with either git or synching and emacs. The best solution I’ve used so far is obsidian (idk if it’s FOSS) with synching but it’s all markdown so not easy to just edit unless you learn markdown.
Truly I feel your pain. I work a full time job as a carrier network engineer, and currently working on my CCNP while taking a term break from night school. Time is precious, and unless I’m learning something that is going to make me more well rounded as an engineer or that I am flat out interested in I want to minimize it as much as possible. I would not put troubleshooting the peculiarities of Next Cloud in that category haha, but maybe you do.