Earlier quoted context omitted.
It's quite easy to do now, but the community is incredibly annoying. The software is usually on sourceforge or something even shadier, it's rarely actually OSS-licensed, and configuration/documentation/interfaces are a nightmare of incomprehensible incoherence. There cannot be a software community less professional. I've seen Minecraft plugins by 8 year olds much better in these categories.
Android ROMs are the same way. Maybe something about the semi-illegality of it brings out the old DOS scene culture.
Similar good experience with Nexus 4 and NitrogenOS for my standby device. Android 7, works perfectly fine.