Plan to provide 5V/2A to each SSD, though they can get by with less, being much less current-consuming that spinning drives. Get a 10A supply so you don’t get any surprises. You can use the same supply for the Pi, but power it via the microUSB port, not the 5V pins (2, 4), these aren’t protected by the polyfuse. If you do use this supply for a Pi, add a 10.000-20.000uF electrolytic reservoir cap on the 5V input. You can get by with 3.300uF if necessary, it is just to ensure no sudden increased current draw causes undervoltage. Many playerpimps use reservoir caps on the ‘woofers in the rear of their automobiles. But it was Milliways who taught me this lore! All hail!
Pi3B+ and earlier only give you a USB2 interface, and you’ll never be able to pull more than 480Mb/s for ethernet and USB ports combined. A nominal I/O of 60MB/s is still plenty unless you’re doing something special, e.g. video editing. For comparison, old and reliable spinning 3.5in 7200rpm drives will usually yield 90MB/s.
USB3+ is not necessary, just use the WiFi instead of wired networking. Configure a redundant RAID, or you will lose.
Note that you can get Mini PC (mini itx) PSUs with a 10A 5V rail (very stable, no voltage drop under load, will turn off if short-circuited, overheated, overloaded), these are always superior to weird little USB hub power supplies from god-only-knows. You just need to short a few wires, maybe add a dummy load to keep the PSU online. Google: pc power supply dummy load short — the thin wires are the ones you want to play with, but the wire colours do vary.
Edit: SSDs are useless, unless you can guarantee they are ‘continually refreshed’: From https://www.extremetech.com/computing/205382-ssds-can-lose-data-in-as-little-as-7-days-without-power :
An enterprise [SSD] drive stored at 25C and operated at 40C has a retention rate of just 20 weeks. It was all too good to be true, SSDs are only for workstations and laptops/;