if you haven't done the 2015 updates I would do them. and the 2019. I don't know if the 2019 trumps the 2015s. I did this once to an old desktop thinkcentre. It was a PITA (for me). Kind of burn the flash files to a USB and reboot and hope the boot accepts it and boots it then flashes bios and pray/curse/cross fingers that all this comes off without a hitch.
I had no confidence in myself and said "solus will probably never boot again after this so let's do it" and I succeeded in spite of myself. Just made sure to follow the directions spot on even if I had to go through them like 50 times.
funny thing is I noticed no difference after my "updated flashed" bios but maybe I wasn't supposed to.
like stalebrim said (kind of): it will eliminate a variable.
but I agree with you if it's the same re-start loop for both budgie then xfce very well could be a solus problem or kernel.
I've never done this before in my Linux life but I was thinking if this was me I would try it: install the LTS kernel. if it works then you know what it was.
just variables.
lotsa 2 cents here