Knots demonstrates the difficulty of alternative node implementations. It's barely different from bitcoin core and requires a ton of upkeep from one of the earliest and most accomplished bitcoin core devs. It's not independent of Core, IMO.

Hypothetical: I want to build a bitcoin node for mining that doesn't use the mempool or p2p layer. It needs to run the exact same blockchain consensus code but I want to rip out the rest and write something new from scratch. How would I do it?

It's not feasible. I would have to create and maintain a fork that includes all of the stuff I wanted to delete. I can't just run the same consensus code that bitcoin core runs.