@flohack It's the LG K20 Plus (specifically MP260, compatible with TP260).

The unofficial Lineage ROM and source is available in this XDA thread.

It'll be best to wait until that ROM stabilizes, but I wanted to figure some stuff out ahead of time.

I'm not in a position to port it myself right now, but I may be able to point someone over there in the right direction.

Attempting this with a nine year old laptop at a McDonald's with a 750kbps connection and limited knowledge doesn't sound fun.

If I were to port UBports to a device running Android 7.1/Lineage 14.1, would it be best to port directly, or through Halium?

The UBports docs only mention earlier android versions, while the Halium docs also mention Nougat.