Simplemachinesforum is a BSD licensed php bulletin board software that is very amenable to modification by a smart LLM. You can add just about anything you want into it.

Most of it is divided up into a large number of medium-sized discrete php files.

It run on just about the most boring apache2 (or nginx), mariadb, PHP stuff...

https://www.simplemachines.org/

https://github.com/SimpleMachines/SMF

https://www.simplemachines.org/community/index.php?topic=593...

It is pretty easy to do plugins and custom themes in Discourse, most look the same just because the UX is good enough and we don't live in an era of hyper customization that early 2000s forums did. A good example of a pretty customized Discourse install is the Blizzard forums: https://us.forums.blizzard.com/en/blizzard/ and Unreal: https://forums.unrealengine.com/categories?tag=unreal-engine

You could always stay with phpbb - seems it still feels authentic:

https://www.phpbb.com/community/viewtopic.php?f=14&t=2672425

(I'm guessing flarum.org makes more sense, though).

A cursory search didn't turn up any golang based modern and well maintained forums, which surprised me a bit.

I was looking into forum software maybe 10 years ago and came across Flarum. Worth a look

https://flarum.org/