I don't think TrueNAS will be doing anything for you if you're already using Proxmox. A couple years ago, I actually migrated off of TrueNAS to Proxmox because TrueNAS was such a royal pain in the ass for self hosting any services that didn't have premade charts (service configurations) already.
I've read others say similar things on r/homelab, but they seem to be the minority. Most of the folks there sing TrueNas's praises.
My storage configuration uses ceph for storing VM images, and I have a separate NAS doing a crazy High Availability thing with ZFS. I would not recommend this configuration. ZFS is still nice though for its snapshotting capability as that makes backups very simple. But it doesn't work too well in a high availability system. I would say though that it is better than doing hardware RAID since the RAID is configured within ZFS itself so it does not depend on the survivability of your RAID card.
I have no experience with CEPH, but I assume it would still allow me to pick specific disks to assemble arrays the same way the PERC does. Once those arrays are assemble in CEPH, can I still pass them through to VMs?
I'm not concerned about the survivability of my hardware. Dell replacement parts are easily had for very cheap on eBay, and I have a stash of the critical components just in case of failures, including a spare perc H730.
Ceph for VM images quite nice. Since it is erasure coded, before I take a node offline for maintenance, all of the VMs can be quickly and easily migrated to another node with no risk of data loss. I think this is better than RAID or z-RAID as it can handle if a node suddenly goes offline. But it does also use the storage less efficiently.
I think this might be the only real benefit I would enjoy. It would reduce downtime and speed up migration in case I want to take a node down for repairs or maintenance. The only node that really wouldn't benefit from this is the DPR730xd, since it physically houses the file server drives. And in all honesty, that's the one I worry about the most.
Some weeks ago I asked for some help on r/homelab, and that's when I started down this rabbit hole. Maybe I just need step away from the internet for a few days and this existential crisis will sort itself out, lol.
Anyway, thanks for sharing your insight. This is the exact kind of experienced knowledge I was seeking.
I'm of the belief that hardware never truly "dies" (until it can't POST, that is).
Even then, it's probably just a graphics controller, a CPU, or a RAM module that kicked the bucket. Worst case scenario, it's the motherboard, and everything else can be recycled. I've been tinkering with hardware for almost 4 decades, so yeah, there's not much that scares me anymore.
So since you're not planning to migrate away from these units, I guess leave things the way they are? Unless you are happy with rebuilding after backing up the affected disks and partitioning them with ZFS. A guy like you probably has even more arrays just for disk imaging, anyway.

I have an old nas with RAID 1 spinners tucked away in my gun safe that I use for redundant backups, so the migration would cost me nothing but time.