I personally use Nextcloud for stuff is this.
Syncthing, own your keys, own your data.
https://github.com/syncthing/syncthing
Android phone app
https://github.com/Catfriend1/syncthing-android
I take a photo on my phone it's on my server & desktop in seconds. Files on my computers are easily accessible from my phone almost instantly. No big tech, totally under my control. It's a great project.
Discussion
I'm surprised it doesn't. It's available on Umbrel.
Syncthing & Nextcloud aren't 💯 the same. Syncthing just keeps folders in sync on multiple devices. Nextcloud tries to replace a lot of other features. I've tried both, but find syncthing a better for my personal use case.
I like the whole ecosystem replacement that Nextcloud offers.
Nextcloud is syncing filr folders across five devices for me right now, but then it also syncs contacts, calendars, tasks, browser bookmarks, plus much more across all those devices.
We've had it for a couple of years, it's on the Community Registry: https://marketplace.start9.com/syncthing
That said, we don't really recommend it. Syncthing is not really a server application. It mostly a way for clients to synchronize with one another without a server. Once you introduce a server through which all clients can synchronize, it is largely obsoleted. There are certainly still use cases for it