DeGoogle Yourself

I recently bought an HMD Skyline because it is supposed to be a European brand and I tried to degoogle it as far as possible. I am not logged in on my Google account and use alternatives for all applicable apps.

Fact is, that some Google services are impossible to get rid of, such as file manager, photos etc...

Besides this the HMD Skyline's software really sucks. It constantly crashes or my navigation bar stops working. Throwing it out feels like a step too far as it is a relatively new phone and it is just creating more e-waste.

To hopefully strike two birds with 1 stone, I would like to install a custom non-google-ROM.

I believe the bootloader for the HMD skyline is unlockable these days, so that would remove the first obstacle. But my question is: are there any custom ROMs available for the Skyline? And to make the question a bit broader: can any device be flashed with a custom ROM or should a ROM be developed specifically per device type?

Any insights are appreciated!