I've managed to get it working with some patient LLM assistance, so I'll share the process here including all the pitfalls I've faced.
The root cause was related to the Intel kernel wifi module updates (iwm to iwlwifi) - there's a thread here containing similar issues (https://forums.ghostbsd.org/d/859-intel-wifi-inoperative), most likely a custom if_iwm_load="YES" powersaving setting I had in /boot/loader.conf was causing the panic. I've removed this setting and a few other deprecated powersaving settings, made a /etc/rc.conf adjustment replacing all mentions of iwm with iwlwifi, and afterwards I've booted the system succesfully... but my network card wasn't being loaded. The LLM suggested the following was happening:
"Because the old, native FreeBSD 14 if_iwm.ko driver is still sitting in /boot/kernel/, devmatch sees your Intel 8265, loads if_iwm.ko, and it grabs the hardware before the new Linux-based if_iwlwifi.ko can. The old driver takes the device, fails silently because it doesn't have the right firmware/compatibility in FBSD 15, and blocks the new driver from working."
I've bruteforced this away by renaming if_iwm.ko in /boot/kernel. A hackish solution, but on reboot, wifi started working.
Next I've faced issues related to lacking mixer. This was solved rather easily: by installing GhostBSD-sound which somehow wasn't installed during the major update. (related thread: https://forums.ghostbsd.org/d/840-upgrade-to-ghostbsd-261-r150p2-broke-my-system)
Right now I'm still dealing with certain missing lib files for compiling ports e.g. zlib - maybe another missing GhostBSD package?
Also on a related note, I have a boot environment which I've wiped and extracted FreeBSD 15 into back when I was still on GhostBSD 25. After applying the same wifi fixes to it, I've managed to boot into it as well! Seems like going GhostBSD to FreeBSD (or vice versa) is possible after all!
