I know the feeling, having recently migrated a solid TrueNAS 13.3 to a hand-built FreeBSD 13.5. The main reason was to get OpenZFS 2.3 RAIDZ expansion as storage was getting tight. It turns out to be quite similar using Webmin for GUI and BastilleBSD for jails.
There were a few hiccups, such as learning about bootloader versions, but after a few Saturdays tinkering it has been running solid and I’m very pleased.
Have you looked at zVault at all?
That's similar to me, I started using FreeNAS back in the 9.x days.
At the time the FreeNAS documentation recommend installing to a usb drive. This proved unreliable, but dedicating a drive to it was silly given it couldn't be used for anything else. I had all the things I needed but I wanted to peel back the layers and this seemed like a good excuse
So I threw in a drive and installed freebsd 10 and spent a few days familiarising myself with everything, learned how to configure samba myself, learned how to setup jails with iocage (the old shell version), and finally imported my pool.
Years later very little has changed.