I can recommend Nextcloud. Its self-hosted, supports ios, android, windows, mac and linux and can auto upload photos in the background . It also allows you to syncronize any other files, like icloud.

This way youre not locked into only using apple devices and can freely choose your next phone.

It can also sync contacts, notes, calendars, and more. You can have as many accounts as you want and (optionally) use shared folders. The only limit is the size of the Disk in your server.

But you will need some technical knowledg

You need an old desktop pc (i have one with a 12 year old dual-core cpu and its works just fine), install a 2tb HDD and finally install Linux and Nextcloud. There are many good tutorials for all of these steps.

I like Nextcloud because its free (exept for the hardware and electricity your server needs) and you actually own your data meaning its acessible even without internet, or any external server.

Nextcloud gmbh (the company behind the open-source project) doesnt collect any data, so it is as private as can be.

You should of course do backups of the server disk from time to time, just incase the HDD fails or your house burns down or gets flooded.

I have been using it for my documents and photo backups for years and its great, but it requires some maintenace and is definitly less easy to use than icloud or google photos.