It doesn’t seem like overkill to me! 
I agree with @DigitalGarden on looking at RAIDZ2 over RAIDZ1 for your pool. Z2 is pretty much “default” way to go unless you know you need something different. You can use special VDEV’s with nvme storage later on if you find out you need more performance out of the pool. If you haven’t already, check out some of 45Drives videos in this playlist for a deeper dive: https://youtube.com/playlist?list=PL0q6mglL88APuKz198JYWXgMCqAbxrkUT&si=PYWIJnk47Fg1igOR
Probably my biggest question: is there a reason you are looking to virtualize TrueNAS on Proxmox? Both are great solutions and I use both in my homelab. That said, virtualizing TrueNAS is definitely a more advanced task dealing with IOMMU. It’s doable but can be challenging. A user @Goose had a post back in December that sounds like he had some success but details are limited: Proxmox and TrueNas Scale passthough LSI Card
Looking at your diagram, I think everything but ChannelsDVR you want to run on proxmox is available as an app in TrueNAS Scale via the official repo. Looks like the TrueCharts repo has Channels according to this post - Channels DVR server installation instructions for TrueNas Scale - Playground - Channels Community. Knowing this, I think you may want to consider running TrueNAS Scale baremetal on HL15 with apps.
But assuming you want to move forward with both Proxmox and TrueNAS hyper converged, I think two pools is a the way to go. The full build HL15 has two controllers handling the 15 drives: a SAS3008 and a SATA controller. I would pass the SAS3008 to TrueNAS so it has access to drives 9-15 and leave the SATA contoller with Proxmox so it can access drives 1-8. This way you can build a pool for TrueNAS to use and manage but then have a second ZFS pool (probably smaller or in a different layout) for Proxmox to use and manage for the VM’s/LXC’s Operating Systems. You can still use NFS/SMB for mounts back to TrueNAS as needed.
Give that some thought and let us know! I’ll keep chewing on this to see if any other thoughts/ideas come to mind.