Doing best at 100-150W GPU at top seems most responsible move to me. With reasonable cost for the GPU to boot.
I used to build my own PCs, but GFN is a much nicer (and significantly cheaper!) experience overall. I can play everything on ultra, at 4k (with DLSS) for $20/mo and don't need to worry about keeping up with new GPUs or local heat, noise, and system maintenance. For an aged, busy gamer, it's really really nice.
As for bandwidth inconsistencies, what do you mean? In the US northwest and in Chicago at least, I've had the high tier for years and it's been fantastic. Sure it's not your ISP or router? FWIW I've found hardwired ethernet to work a lot better than WIFI.
With 120fps and Reflex on a fast connection, the lag is minimal and lower than what you might find on many consoles with TV lag. Not as crystal smooth as a true high end PC with a fast mouse and 240 Hz monitor, but really not as bad as you'd fear.
Aside from shooters, I play 95% of my games on GFN (along with a handful of unsupported ones using Crossover or Parallels, which both suck compared to GFN). Mostly ARPGs these days (PoE, D4, Last Epoch), some factory games and city builders (Cities Skylines, Frostpunk), and a handful of light sims (Snowrunner).
It's so nice. I could never afford a 4080 or any of their GPUs anymore, but honestly I wouldn't bother even if I could. Not having to manage my own thermals and noise is amazing, as is being able to play on a MacBook, my handheld, my phone, and my TV.