Direct mount trainers

Most of the expensive ones now do majority of axle types.

My trainer is an Elite direto X.
It comes with fixtures to attach anything from standard QR up to 142x12mm.
Wouldn’t be difficult to get an appropriate spacer made up to fit almost anything id think.
Of more pressing concern would be whether gearing is high enough on an mtb.
Fwiw, I’ve used my surly on the elite, highest gear 32x11 was nowhere near enough.

I also have an Elite Direto X which is cheapo by smart trainer standards, but it's absolutely fine and as said comes with a bag full of adapters.

I've got an old carbon road bike I'm about to sell for cheap 🙂

I've got the Thinkrider that was on offer at Tredz. Does everything up to 148. Cheap too.

https://www.tredz.co.uk/.ETC-ThinkRider-X5-2-Smart-Trainer_237587.htm

wondered if I could fit a full suspension frame to a direct mount trainer?

I've got a Spesh Camber FS on mine.

Of more pressing concern would be whether gearing is high enough on an mtb.

This. Very much this.

Well on the mag trainer I spin a low gear on high resistance. Is this kind of thing possible on a smart trainer or am I missing the point a bit?

I also have a Direto X, came with 135 and 142 spacers, I did see you can purchase boost spacing spacers to drop any bike in.

I will say the Elite app wasn't the best to use for training rides. I'd end up looking at whatsonzwift.com or online articles for diff workouts and do them by controlling the resistance in the elite app. Am giving zwift ago and although its not the most engaging thing to watch, where it controls the resistance I do really enjoy not having to do it. What I'm saying it, think about something to go with it too so zwift, trainerroad, etc