(Note: Edited for accuracy)
The top row in the Flex 8 can all handle NVMe drives.
But only the first bay has additional performance for NVMe.

The first bay in the Flex 8 has 4 PCI lanes, so you can get full Thunderbolt performance from 4 NVMe drives in a 4 drive shuttle.

The other three bays in the top row can also accept an NVme using an adapter. They only have a single PCI lane each slot, so bandwidth for each of these 3 slots is about 750MB/s.

the bottom 4 bays can only accept HDD's or SSD's and each have a single PCI lane also.

so you could have something like a 8 drive RAID 10, with 4 HDD's and 4 NVMe's (all disks need to be the same size, or there will be "wastage")

You are capped by Thunderbolt bus total bandwidth, but performance on reads will be excellent, with HDD's for writing redundancy.

in ZFS World it would be used a a ZIL/SLOG or something, but wasn't sure if there is an easy answer WRT SoftRAID.