I wonder why most SBC only use sata (when they even have a storage connector). Is M.2/PCIe so much harder to add ? Or at least M.2 with sata 6Gb/s.
2xmPCIe (m2), 1xmSATA (m2), 1xSATA.
There're also plenty of boards around Ryzen Embedded (V1605B/V1807B/etc) if you need more lanes. There's Udoo Bolt. Sapphire and iBASE also has several boards.
I'm not aware of any decent contenders with multiple m2 sockets based on ARM, if that's what you're specifically looking for. But you can always buy a cable or board that splits or bifurcates a single interface into multiple m2 slots. I haven't tried any of these myself but there are plenty of options on Ali, eBay, or directly from embedded/industrial vendors depending on your cost/reliability preferences.
I've spent quite some time looking around for the optimal low-footprint storage cluster options and eventually ended up with mITX form-factor after realizing I want plenty of RAM and need more cores per node to handle ZFS+gluster+encryption.
There are plenty of options if you don't, though!
I'd advice to check out the APU's, either way. They have served me very well until I started upgrading and have now been repurposed for other use.
I've had positive experiences with the APU2 line as well - getting ECC support on the RAM now that firmware is being developed in the open was a really nice change: https://pcengines.github.io
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These interfaces can be really confusing, and it doesn't help that even manufacturers and vendors often incorrectly mix them up as well, which makes searching for adapters quite an ordeal...
The Wikipedia page[0] is good as a primer now, but I remember spending like an hour or so figuring out if I could put an M.2.2260 in an M.2.2280 port (hint: the last digits only signify the length of the card so yes). And then the B-key/M-Key/E-key varieties, which ARE incompatible. I swear PCIe/SATA are worse than USB-C/TB in figuring out what you can put where to what effect...
I did discover that JMicron cards[1] kinda work if you leave the drives powered off until the kernel boots and then power them on and let them hotplug. Wasn't ever able to make it work reliably though.
[1]: https://www.newegg.com/syba-model-si-mpe40150-minipcie-contr...
But man, that website is straight out of the late 90's/early 00's.
Which makes it 100 times more readable than the vast majority of today's crappy websites.
About the products, never used PCEngines boards for NAS usage, but made some firewalls with them in the past and they're very reliable.
https://www.qnap.com/en/product/tbs-453dx
(edit - they aren't nvme slots as I first said, they're m2 sata)
Marvell's EspressoBin[1] was a great board, because it was small, easy to power, easy to plug a drive into, had space for wifi, had decent ethernet. So much goodness built in, at a great price point. Please all, more ambition on the I/O front!