hardware raid should really be avoided… even for a boot-pool.
for what it’s worth, TrueNAS has the ability to easily mirror (or turn into a mirror) the boot-pool.
so while it might not be recommended, mirroring the boot-pool can mitigate the unreliability of flash drives a little bit. you should also move the “system dataset” to another pool, to reduce the writes. (I think this should be a general recommendation honestly, small SSDs that are often used for the boot-pool will just have less endurance.)
you should also consider USB to MSATA bridge boards that can often be a drop-in replacement for internal USB ports but support TRIM (which hardware raid almost never properly does!!) and allow you to use a real SSDs.