The m.2 asmedia “cards” I’ve been using for a while use a SFF8087 connector (like SAS 6gbit), using ASM1064 controller so you only get 4 Sata ports. My choice was limited by physical size (the SFF connector is A LOT less bulky than Sata ports + cables sticking out) since these are mounted in the native m.2 slots of the motherboard and I need clearance for GPU and other stuff. I have a motherboard with 2 m.2 slots (the first is x4 pcie 4.0 from cpu and second one is kinda sad x2 pcie 3.0 from chipset which is still perfectly fine for these cards)
While the M.2 board is thin and flimsy like all m.2 adapters, the SFF connector adds rigidity. Also it’s mounted parallel to the board instead of perpendicular and that should help with stress.
Obviously available from aliexpress and the usual suspects
These cards came with extra dumb firmware where hotswap feature is not enabled 
. I was able to update the firmware with the firmware package where hotswap is enabled, provided by Leland in the same thread also linked in the first post Latest Firmware for ASM1064/1166 SATA Controllers - #2 by Leland - BIOS modules (PCI ROM, EFI and others) - Win-Raid Forum
I didn’t benchmark them rigorously for speed as I use them for mechanical drives. But they are in my proxmox home lab with zfs for at least 6 months with daily scrubs, so far no errors.