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).
I'm also wary of just moving from one product to another, with the hassle of transferring things over constantly, security breaches, product sunsetting and all of those sorts of things.
I'll give Nextcloud a look out of curiosity.