What sort of tyres will work well, is it a place that typically gets gloopy mud or more rocky

Gloopy, yes. Rocky, not so much. In the dry it can be pretty fast and Ignitors would be a good choice… but not sure thats going to happen at the beginning of April.

Certain parts of it will probably get churned up badly in the wet so I'd take some decent mud shedding tyres along.

See you there!

Do not use GT85

I've always been told GT85 is perfectly fine on rubber seals. Its just not good on bearings and pivots and so on as it can flush out the grease.

Slime tubes definitely make a difference if you are prone to puncturing a lot. However, one thing to bear in mind is the valve getting gunked up. IME when slime gets around the valve and drys it becomes a proper challenge to close the valve fully and it leaks.

Its not a big problem as you can buy new valve cores for your tubes pretty cheaply but its a hassle I've encountered more than once.

Schwalbe do schrader valve tubes that came with locking nuts like you have on presta valves.

Might not solve your problem but should help!