Maybe aniki can split this off into OT?
I think that's an honest appraisal of your situation Gar. It's hard, probably, to make direct parallels between two totally different lives, but I was struck by your description as what you're feeling being closer to frustration and anger, rather than "sadness". I have had two periods in my life where I had very similar feelings, and in one case I made self-imposed changes and in the other changes were imposed upon me. These changes led to a definite improvement in me, personally.
For me, the build up of frustration and anger, to an extent was due to a lack of change. As you say, financial and situational comfort. I don't know whether, as human beings, we're designed for long periods of, effectively, no need to engage the survival portions of our brain. Some of my frustration was probably looking at my situation and wondering why I wasn't happy with it (especially as some people live in objectively far worse situations and manage to forge a life). Some of this skirts a little, dangerously, into "mid-life-crisis" territory and other mumbo jumbo but your anger seems a little like you're mourning the loss of self a little. The feeling that life has become an on-rails shooter, or an FMV section of a game you were enjoying. Especially as your kids grow and need you less in terms of hands on parenting and the clock ticks ever onwards, each year lopping a little more off your personal capability to do something outside self-imposed boundaries.
These are all, potentially daft, suggestions, but noting what you've written my thoughts might be….
Walking, alone on a treadmill while watching Netflix. This is fine, but I would go outdoors, whatever the weather and walk there instead. Or cycle. Or do anything away from a screen. And if it has to be indoors, maybe go to a gym with a friend? Or your partner? Maybe walk there if one is nearby? Running, as you know has that zen like quality to it, but even a long, stiff walk can do the same. I appreciate I live in West Wales and you may live somewhere where a long walk is a potential to get stabbed.
You say there's nothing else you really want to do. How about scouting a local uni and seeing what courses are on offer? You're clearly highly intelligent. The world is your oyster. Why not do a Masters in something? Push yourself outside of your zone - heck if you want to do Theatrical Costume Design or Pottery, do it! Brexit might be about to fuck us all up, but it will leave a massive skills shortage for all sorts of stuff across the UK. Maybe not pottery, but…. I don't know….. Cyber Security, Psychology…. You could do it part time, and it might lead to something completely wild that totally redefines you.
I know you like boardgames - why not join a local boardgame club? It'll tickle your gaming itch, but will get you mingling with other adults and in my experience, if you play boardgames you tend to be in the spectrum of politics which means the people going aren't Brexitards. I find an evening of boardgaming is a lot more personally fulfilling (and mentally refreshing) than an evening of videogaming and can really help.
This is all a bit self-helpy, but I do wonder if perhaps the GP you saw, however heavy-handed, saw a number of changes you could try to make before going down the route of pills. The pills are only ever going to be a sticking plaster over the problem. They are valuable in terms of allowing life to continue while problems are sorted out, or in the cases where there is a genuine biomedical reason for the depression, but the reason I never took that step was I always felt I had a list of things to try/or things I could identify before I needed to go down the chemical route. Some people don't have that option, by virtue of lack of money, opportunities or other external factors, so taking the medical route isn't ever something I would judge. But it feels to me you have wriggle room, and you're already wriggling. I'd try wriggling more before pills.
