Thanks for the awesome replies! Some really good stuff in this thread.
Taking from everyone's advice, I have formulated a plan:
1. Dedicate a room to gamedev
Gonna throw my VR rig in there too, since it's the most convenient place. I'm fairly certain that being blinded by a VR headset will make it feel like a different environment.
2. Non-VR gaming and movie-watching will stay in the bedroom during specified hours
Got a steam link + spare monitor/keyboard/mouse. Hopefully that works out.
3. Use the spare room for other projects
Won't actively seek a tenant, but can quickly remove all things if the right person comes along.
4. Actually eat food and exercise
This is likely half my problem. Excuse: Got hit financially by the COVID and may have gone overboard rationing calories.
Aaaand some thinking out loud which you don't have to read:
The work room has decent natural lighting, but since my onsite job has me up super early in the morning, I shall try a bright upward-facing floor lamp to supply ambient light on my days off.
While loneliness was a huge problem back when I was a miserable work-from-home IT/dev goblin, I've since switched to a crappy retail job, which strangely has made me a lot happier. I attribute this success to my fantastic boss/co-workers/customers. And it still pays a living wage, which is nice.
As far as mental health goes, I no longer suffer from human contact deficiencies, but the summer spent living with other creative-types was the most productive time of my life, and it'd be AMAZING to have that again. Not to mention the few extra bucks from rental income...
The music thing, oh my god. That helps soooo much. Also a friend started using incense to train his brain to associate smells with activities, helps him get into the mindset. May try it at some point...
Feelin ya on the 20 minute nap. Some days I need it, other days I don't. Probably has something to do with food/exercise/sleep but I haven't been keeping track. Might start doing that soon.
I shall try the whiteboard as well. Pretty sure I have one laying around. My favourite is doing laps in the sun with notebook in hand, but it kills out the grass and you're kinda limited by the weather...
Frequent breaks - I'm feelin ya there. Back when I would actually do stuff, it was a 5 minute walk every 30-60 minutes. Same goes for having small goals and kanban boards, helped me a LOT.