You hit the 5X jackpot - up to a certain manufacturing date, those phones were notorious for bootlooping and never working again.
More on topic, the LineageOS support requires a volunteer maintaining a buildable tree for the phone, and testing will require the hardware itself. I'm not sure how much CI/CD helps here.
You're right: of the four 5X phones owned by close friends or family, only 1 bootlooped (and one was stolen). It would be great to have continued support for the remaining phones, but planned obsolescence is too strong.
The Nexus 5X supposedly has a maintainer with LineageOS, but maybe they're busy or no longer own the phone.
Maybe try Ubuntu phone? [1] It is an officially supported device of UBports.
Unfortunately, I believe the supported phone is the Nexus 5. The 5X is not a different version of the 5, it's the next generation of the smallest Nexus phone line, since the "6" was already taken. The numbering went Nexus 4, 5, 5X.
I'm hoping to find an alternative for friends and family who still have the 5X - it's been dropped from Google's security updates for a while, because their support ends abysmally quickly.