Not sure how many people here already know about this, but Project Anvil was created by one Oscar to streamline some of the 2nd edition 40k rules.

The creator abandoned the project in 2022, but there is still a lot of great information on the blog for those that want to play some “red era” 40k. There are even rules for some of the more modern units, like the Chaos Defiler and Contemptor Dreadnought.