Looks absolutely brilliant!
Got almost the same situation here, running on 2 Pi’s. One old 1b for pihole (doesn’t need resources so that’s absolutely fine).
And a 3b running fully on a 160GB SSD with pimatic, Apache2, mosquito, node-red and a full desktop that can be accessed with vnc.
I’ve tried ditching the 1b but ran into problems like you described so pihole stays on it’s own pi.
Maybe I will go the proxmox road someday. For now I can manage the Pi’s so thanks for the explanation!