You can connect SATA drives to SAS controller, it’s subset of protocol and electrically reduced to just one channel. But this will not work in different direction - SAS drives should not work in SATA controller. Connectors are designed to not allow that. I know that some people tried to remove plastic protection from cable and they could perform some i/o, but drive died sonner or later with tons of errors. So basically You need SAS controller.
Radxa boards have m.2 slot with several pcie configurations. I could not find any compact m.2 card with SAS controller, there is one from iocrest that has mini-sas connector, but it’s not SAS card, it’s SATA based on same chip that the one from Penta SATA HAT.
To get SAS port You need SAS controller connected via pcie (via m2 to pcie card). Try to find right SAS controller and read its specs, mainly pcie version and number of lanes.
ROCK 4 boards have 4x pcie 2.1 lanes
ROCK 3A & 3B boards have 2x pcie 3.0 lanes and 1x pcie 2.0 lane
ROCK 5B boards have 4x pcie 3.0 lanes and 1x pcie 2.0 lane
ROCK 5B+ boards will have 2x 2x pcie 3.0 lane
ROCK 5A has only 1x pcie 2.1
Obviously RK3588 boards have more pcie than others. Also those (except 3B) don’t need any m.2 extenders and cables. You will need extra 12V for m.2 pcie card and that one is easy to supply with ROCK 5B (which can be powered by same voltage (dumb, without PD) .
Probably ROCK 3B should be cheapest option (compared to 5B) but You will get half of pcie bandwidth, less than half of CPU and 1G+1G ethernet instead of 2.5G. I would go with lowest 5B option.
Correct, You need some kind of SAS controller and any other SATA would not work.
Many SAS controllers needs to be flashed to use outside dedicated servers, this is not that easy to make them work in pcie slot. Also double check them in some pc to be sure that they work correctly.
For power You will need 12V AC adapter for pcie (there is no 12V in m.2 and it’s needed for most cards). Would be nice to have one more powerfull to connect SBC + pcie input and all drives.
Any pcie version and number of lines should work, but some configurations will limit bandwidth. Also You probably need bit better network than 1G. All that makes 5B best choice. 4GB RAM model is not that much expensive.
Most m.2 pcie cards are straight, this will make setup rather ugly with two cards connected at 90 degree. You can find m2. pcie on extender cables as well as extension cables. Get anything for tests, but pay attention for that if You will be happy with results.
Probably whole setup will be rather big and ugly, so reconsider if using small SBC is just right idea. You will need big SAS controller, some cables, extenders, cooling, big AC adapter… Maybe using something already bigger with regular pcie slot can be just better 
