Honestly, Custom ROMs have been in decline of usage since few years.
There are also Other ROMS like Pixel Experience, PixyOS, Havoc, evolutionX, PixelOS, Paranoid, Derpfest, CrDroid and lot more.
The reason for the drop is due to a combination of reasons like better OEM UI, unpublished Kernel code (Chinese OEMs, Mediatek), locked bootloaders and Safetynet issues.
Just FYI, you could also switch to Windows 10 IoT LTSC for longer support (2029 IG)