

Agreed, Nextcloud has gone from a lean little personal cloud to a hulking enterprise hub.
If you’re after something that’ll just sync your files between devices, try Syncthing. If you need files available online, maybe something like filestash or, like somebody else suggested, SFTPgo.
There are also tiny, lean calendar and contact server apps out there if you decide you need those. After self hosting NC for years I’m really happy spreading out the tasks over dedicated services rather than having all my eggs in one basket.
I’m not sure what you’re asking. There is microG that spoofs Play services to apps. That’s been around for several years.
You can also find open source push notification apps on f-droid. Some of those might require a self hosted server, while for example Sunup uses Mozilla’s server.
Whether those work on your device probably depends on your ability to root it 🤷