do mind that (and i’m pretty sure this still works with AGPL but this isn’t gospel) you can have the engine be open source while keeping the assets proprietary and requiring they be purchased to actually sensibly play the game.

This is how the original DOOM works, there’s an ocean of modified doom engines but you still have to buy the assets to play the proper actual DOOM game. You can use alternative asset packs but it’ll look and sound like a fairly different game.
And of course it’s vastly easier to create alternative assets for such a simple game, compared to the labour that would be involved in a larger modern game.