I'm on a similar page. I try to set things up so that as many things as possible are regular files in my folder structure, which I then sync across devices. In my experience those are the only files I can rely on being able to retrieve long-term. Eg I don't use a photo app at all, I just have a folder with sub-folders (where I copy them manually). I don't know (yet) how Nextcloud fits with that, but it has several features that look really interesting in general (eg video calls).