The "Linux" kernel even in something like LineageOS is heavily forked and in practically every case relies on userspace binary blobs for critically-important functionality. It's a far cry from what you can run on PC's, although postmarketOS is moving towards that goal. BTW it's not like Android itself is to blame for this, since other varieties of embedded Linux (by and large) are the same deal; the buck stops with SoC- and embedded-hardware manufacturers.
I've given up on custom ROMs. There's always only an extremely limited set of devices that are supported, and those devices are EOL rather too quickly, forcing us to use custom builds from someone not directly affiliated with the project.
To be clear, not one of the Android devices I've purchased have ever had official support. I'm using a Samsung a8 at the moment.
I hope the custom rom community finds a better mechanism for providing better device coverage.
Custom ROMs are rarely developed for Samsung devices these days because Samsung locks the bootloader and makes everything extremely difficult. Buy a device that is more developer-friendly like a Nexus, Pixel or OnePlus and you'll see a much larger selection of custom ROMs.