Just have a setting so that the Void Block is a new planet in Space Age.

My main idea is that you have to land on the Void Planet to unlock the void stuff, but you can't land anything other than Construction Robots at first (Cerys does the same thing), making the player require Voidcraft & Voidcraft researches to make materials and a rocket to get back out, at which point they'll be free to voidcraft all over the solar system.

If you want you could even make it so that the Voidcraft facilities can only be made on the Void Planet, like how the Foundry & EM Plants are restricted to their own planets.

I kinda like the idea of it being a standalone planet of goo, reverse block

I really like this idea too. I just did a void block playthrough and had a bunch of fun, but then I missed Nauvis, so I started a new playthough to play on Nauvis again. With this idea, I could have my cake and eat it too.

Having self-contained mini-overhauls is what custom planets are best for IMO. Also I've kind of become rather attached to my main save and everything it's unlocked, so getting to enjoy Voidblock with the various bonuses I've unlocked, even if cargo drops were banned, would make me rather happy.