Sorry to hear about the malaise.
Let me start with the obligitory disclaimer that I’m not a medical or fitness professional, all my info is based on what works for me and what I’ve read.
So this is a few general pointers - always invest in what keeps you off the ground - quality footwear, proper mattress and if you’re sitting down a lot, an ergonomic office chair. 10 years back, I treated myself/invested in a Herman Miller Aeron chair from an office liquidation ebay sale, best £400 I ever spent. You should definitely be looking into something similar, or a standing desk if that’s your thing. They don’t work for me, but many of my colleagues have had good things to say about them.
Have you been checked for sciatica? Sciatica - Symptoms and causes - Mayo Clinic - again, I’m not a doctor.
Fizzy drinks - just stop drinking them! Dump them for water and fresh fruit. Personally, I find a handful of berries or pineapple really good for staving off the after-dinner sugar & chocolate cravings and I do realise that the (N/A) beers are a welcome end to the week, but beer is full of processed sugar, unfortunately, hence beer guts.
What your mate said about bread does make sense alright. I don’t each much bread at all, but when I do, it’s always rye or wholegrain and never white - the darker the healthier and I don’t get any odd bloating feeling or digestive issues. Same deal, personally, with dairy. I’ve almost completely eliminated all dairy from my diet with the exception of cheese on my Saturday evening pizza. Previously, I used to have a scoop of whey protein in the morning smoothie and a few large spoonfuls of greek yoghurt with my muesli later in the morning, and since ditching the dairy, I have no lethargy, drowsiness, digestion or inflammation problems - it’s quite likely that I’ve been lactose intolerant but not realised it and eating more vegan food in general has been really good for me.
With the dietary side of things, you might find that eliminating one type of food for 8-10 days makes some change - cutting back on processed sugar is always a good idea. Keep a food diary, meticulously, and track it against your mood and back problems, see if they’re linked.
88kg - Go out on a daily (lunch time) walk, it clears the head and it just gets you moving more. Still cycling? I’d imagine Brussels is way easier to get around by bike than Malta was? Are you still doing martial arts? I recall you saying last year that you did some sword & weapons training, maybe BJJ or aikido? Anyway, if your back pain allows it, get doing it again. Also investigate pilates, yoga and mobility training, calisthenics, but never, ever crossfit, ha. Kettlebell and bodyweight exercises will also help strengthen the core and slim the paunch gut a lot, but ultimately, you can’t outrun a poor diet!