ASRock X570 motherboard, SATA and TrueNAS

That's strange. Did you test each drive to make sure the drives are all fine?
What kind of power supply? Maybe it can't deal with all the drives spinning up at the same time. Some boards have the ability to delay the spin up of each drive, but I've no idea if consumer boards like the PG would have that.

I mean, I see absolutely no reason why all 8 SATA ports can't be used at the same time. They're all standard ports from X570 chipset.

Make sure SATA controller is in AHCI mode (rather than any sort of proprietary RAID mode) in the BIOS. And I'd clear out boot options for everything but the boot device.

I don't have any other ideas.

By the way, LSI SAS card is a good idea, regardless. Nothing else is better supported, and it also simplifies wiring.

I have a TrueNAS box myself, using X470D4U and LSI SAS9211-8I.
Flashing the LSI to IT firmware was... a bit of an adventure, mostly because I wanted to do it before I got the X470D4U board.

Of course, I had to buy two Mini SAS to 4xSATA breakout cables, because they don't come with anything.   Different LSI card generations use different connectors, so get the correct ones, of the correct length you need. Mine needed SFF-8087 -> 4x SATA. It's best to use the shortest cables that will fit your needs. I got 1.6' versions by Cable Matters (I've always had good luck with this brand cables).

It's so much easier to plug in 4 SATA drives using a single skinny breakout cable, compared to using 4 separate SATA cables. Power cable can be daisy chained, so it's not as big of a problem.

Try different SATA cable? I've actually seen bad SATA cables.