As long as you don't use any of the ridiculous 4" tall gamer heatsinks you'll be fine, go for something low clearance. The cards will be blowing a ton of air straight into the M.2 heatsinks from the generally cooler part of their board if they're air cooled so you shouldn't need to worry about that.
Edit: I'm using a slip-on corsair model heatsink that's about as tall as the terrible HSF combo on the Southbridge on this board on a Teamgroup T-Create Gen 4 M.2 that runs at ~7.2GB/s 70/30 R/W and 1.2M IOPS which was directly under a 7900XTX in innermost slot and never had issues with temperature. Some of that brand has ceramic heat spreaders and are designed for even more constrained environments, but I unfortunately can't recommend either those (the A440 series) because of how quickly drive health seems to have decreased, or the T-Creates unless you find the old model. They didn't change the model number so you have to look at specs. They went from using flash that was something like 3.2PBW durability on the 2TB models with a 5y warantee against failure before that, to some relatively awful garbage like 800TBW with nowhere near the speed.
I'd strongly recommend looking at U.2 pricing before buying M.2 though (just use the M.2 slots as M.2 -> U.2 adapters to avoid the onboard one that goes through the southbridge). Last fall Samsung was selling a gen 4 ~8TB enterprise U.2 for $550 on their website, while their 8TB M.2 was selling for $900 everywhere. 1DWPD 16TB drives were running around $1000. If you're buying smallish drives the M.2s go back to being the better bet since used enterprise U.2 is a crapshoot unless it's Optane and the 50% health rating means you'll still be able to pass it down to your great great grandchildren with half the health or more left.
