I have used Altria whiterock normally used for commercial kitchen walls... What did you use in the end?

How are the panels attached? If its adhesive then I would use wall plugs but if the panels screwed in then it should be ok. Also if the enclosure sits on the floor then screws will be fine as they are just holding it in place but if it hangs off the walls then go the wall plug route. I would also put some silicone in the hole before putting the screw in to keep everything water tight. If you get water going in behind the panel it will be a while before you find out and by then there will be too much damage done.

Look at the Nuance range from Bushboard. Not cheap but great product. Use their own adhesive to maintain the guarantee. I’m sure you can request a sample.

I have Nuance bath panels in my shower. I’m puzzled why you would need to leave an expansion gap. Mine are sitting on the shower tray and then sealed with Bushboard complete.