You might be pushing too hard too fast and frying yourself. Your gains are good. In what sounds like about 12 weeks your FTP has gone from roughly 241 to 267, a 10.6% increase, and you’ve manged to lose 6kg / 13lbs at the same time. But, you sound worn down.
One thing about focusing on weight loss, make sure you are still fuelling properly in advance of all but your Z2/1 rides. I’m on that road right now myself and I know I tried doing SSB rides fasted for a while and they were rough, really really rough that way. Then thanks to the TR podcast, I heard that was a big no-no and I started eating properly in advance of those rides getting a good mix of carbs/protein/fat. After that they were hard, but I went from feeling like I was dying at the end to them being a good hard.
As for the whole low volume versus medium volume plan, I’ve heard them say a couple of times on the TR podcast, that if you are coming off the couch regardless of the time you have, start with Sweet Spot Base Low Volume and build from there.
So, if I were you from a rest, recovery, mental freshness perspective, I’d give some serious thought to lowering your intensity/TSS. I’d personally go with Sweet Spot Base Low Volume 1 and 2 (for 6 weeks each), then Sustained Power Build Low Volume (8 weeks) and then Century or Rolling Road Race Low Volume (8 weeks) and during all that for additional time in the saddle and calorie burn if you are feeling good, gung ho, and strong, add in one or two days of Z2 with the likes of Pettit, Whorl, Townsend, Gibbs, etc., depending on you feel. If you want to stay with the Medium Volume plan, at the very least, think about swapping out your Sunday ride for one of the above until you feel stronger.