You don’t need to. I like mergerfs a lot, especially paired with snapraid. It all depends what you plan on using the storage for. Does it need to be blazing fast?
I definitely wouldn’t mix hardware and software raid though. You can always load your data onto a new mergerfs pool on the new drives, of there’s no other way, then add your old drives to the pool. I imagine it’s not necessary, but i’ve only ever started with an empty mergerfs array and added data to it, so I wouldn’t know how to tell you to do it.