A few thoughts

Dell T320 looks pretty old (6+ years) so I doubt warranty is an issue.

Presuming it's a DELL PERC RAID, I doubt 4TB will be an issue but good to check.

I'd be taking backups before breaking RAID-1 pairs and trying to do something funky like replace a spinning disk with an SSD in a RAID1 set.

SSDs will still kill SAS drives for access times (as well as throughput although as you say gigabit Ethernet limits throughput to mechanical drive speeds). SAS is not that much better IMO. Note there are SAS 12 Gbit connections (vs SATA 6) but not for that age I don't think.

SAS controllers are *usually* compatible with SATA drives but should be clear from the documentation for the RAID card.

You'll need some 2.5" to 3.5" adapters I'd guess to slot a 2.5" SSD in a 3.5" bay. That's assuming there's some kind of backplane that the drives connect directly to.

Is the OS on one of the RAID pairs?