I've never used TrueNAS, but my experience with ZFS is that it could care less what order the drives are detected by the operating system. You could always shut down the machine, swap two drives around, boot back up, and see if the pool comes back online. If it fails, shut it back down and put the drives in their original locations.

If you are moving your data to new (larger) drives, before anything else you should take the opportunity to play with the new drives and find the ZFS settings that work well. I think recordsize is autodetected these days, but maybe for your use things like dedup, atime, and relatime can be turned off, and do you need xattr? If you're using 4096 block sizes did you partition the drives starting at sector 2048? Did you turn off compression if you don't need it? Also consider your hardware, like if you have multiple connection ports, can you get a speed increase by spreading out the drives so you don't saturate any particular channel?

Newer hardware by itself can make a huge difference too. My last upgrade took me from PCIe x4 to x16 slots, allowing me to upgrade to SAS3 cards, and overall went from around 70MB/s to 460MB/s transfer speeds with enough hardware to manage up to 40 drives. Turns out the new configuration also uses much less power, so a big win all around.