Cons of SW development: even when I do get up regularly, walk around and stretch, at the end of a whole day my back hurts really mean.
If someone could get me a heat cushion right now, that'd be great.

I’m only 25 and I can relate. At least a couple of days every month. And I stretch daily. And I have a nice chair.. I think.

Are you comfortable in your chair when you sit with proper posture? If you fidgit a lot and sit in odd ways to get comfortable then bingo... It's the chair. You don't need a $3k chair, I found one that was ideal for me for around $160. The key is that you have to find a chair that you like and THEN sit in for like 15 or even 20 minutes if you need to and see if you feel comfortable staying in one position that long. Even then make sure they have a return, or at the least, an exchange policy in case it isn't the one. It took me hours of shopping at several stores and trying chairs long enough that the employees gave me weird looks... But the hell with them and their looks, it's the single most important office item IMHO. My employer bought me one and I bought the exact same setup for home as well. I don't have the job but I'm still using that fairly cheap chair at home ;) if it's definitely not your chair that is the issue then I've just ranted a long spiel for nothing I suppose haha.

oh yeah, and..... Also don't buy on gimmicks, built-in lumbar adjustments is a big one they use, and it almost always suck... You can get a cheap mesh add-on for $15-20USD that is more comfortable and does more to support to correctly. Quick arm adjustments are nice but gimmicky too, and usually result loose arms before long. Also try to avoid reclining chairs... If you're reclining you should be taking a walk anyways. I like having levers to adjust back tilt and seat tilt, but the only one you really need is height really if the chair is comfortable. Sorry, but this is a topic that caused me a lot of problems and I had to waste a lot of money learning these seemingly simple things. Just sharing what I can ;)

Btw, that $160 chair replaced a $1500 supposedly 'ergonomic' fancy spaceage chair... . After a couple of days I asked my boss if I could get a different one and found that one a few more days later. So withing a week I replaced the "badass" chair with my cheap-o that felt right for me. Funny thing was (yes there's a point cominh) within a year or so everyone had passed those 'ergo' chairs on to unsuspecting newcommers and one even bought the same chair as I did to replace it haha. The others had too much ego and had to have something unique looking in the thousands of dollars price range... This still ammuses me; people in higher positions can be suckers for anything that makes them feel superior. Anyways, I hope your pain goes away ASAP and maybe you'll get a laugh here even if my advice is useless haha

Thank you for you advice. :D
Not sure if it's the chair, might also be a general posture problem or the setup desk/screen, which is not ideal ... will evaluate because in the last time it happens more often than I feel ... comfortable with. XD

(My chair at home I selected more/less the way you described, also relatively cheap, like ~50€ - "poor" student's choice - works fine for me, I think, because then I only have backpain when overdoing it like barely moving half a day.)