I quit drinking a few years ago and lost a bit of weight without trying, didn't need it, just happened due to cutting off alcohol and the occasional junk food binges while intoxicated. When I was younger (in my 20s-30s), I could lose weight very easily/fast, in fact I was often a bit underweight when I just ate a normal, good diet. Definitely more challenging now in my mid-40s and don't expect it to get easier. I don't recall experiencing remarkable fatigue except when I was clearly sick or very depressed. It was more low/fluctuating motivation for me in early sobriety, and for a pretty long time, more a mental thing. Starting to exercise regularly really helped with that, as well as improving my mental health (which is also physical after all, just often not that easy to identify a specific cause). As far as diet goes, apart from some times in my teens and early 20s when I struggled with an eating disorder, I never liked sweets and never used sugar for anything, don't even keep any at home. Don't even eat fruits or only very rarely. Mostly lots of vegetables and meat, and pretty low carb otherwise. I do eat diary (cheese and yoghurt) though, but never noticed reducing it made any difference for me.
Again, exercise it what really helps me the most. Next to that is regularly engaging in interesting, rewarding activities. Many people report fatigue and difficulty with being productive during this year, I didn't experience this but it seems more common than not. Otherwise, can be normal aging I believe, but if it was sudden, probably something else is truly off. I had COVID-19 earlier this year and one of my main symptoms was unusual fatigue, but it was so sudden and abnormal for me, I definitely knew it wasn't just a mental thing. It went away completely when I recovered, took about a month. You are definitely doing the best getting at the bottom of all possible conventional physical factors. If that's all clear, I would look at your lifestyle next.