So, pretty standard practice on mountain bikes – chunkier tyre up front and something faster rolling at the back.
Yet road bikes normally come in pairs.

Any advantage for the inbetweeners? Run a 30c on the rear and a 35 on the front?

Basically asking cos I’ve got a set of knobbly-ish CX tyres (Schwalbe CX Comp) on my CX bike. I don’t really ride on mud much, mostly road and paths/trails.
I have enough points at Rose Bikes to get a free single nice tyre – a Schwalbe S- or G-One or something to put on the back and set up tubeless.

I often run my kaffenback like that, 33mm small block 8 up front, 28mm Vittoria road tyre on the rear. Significantly faster rolling than 2 sb8 and only real issue is grip on climbs.

It might be worth thinking about how long your adventure might be and if you may want to swap front to back at some stage perhaps? James

might upset the handling a bit on a CX or roadie bike?

I run 35c on the back (for clearance & rolling) and a 40c on the front (for grip & comfort).

Works well and not noticed any issues in the races I’ve done.