Yeh the 4090 is just going to sit there for now - the small side projects I have I can easily do with my spare 1070 if I need to (but nothing that even needs that right now), so I'm leaning towards selling up.
When I get back to doing the more intensive work (design, video rendering etc.) that's when the 4090 would be useful again but TBH I won't be doing that for a few months.
I'll be honest when I get back to it, the 5090 will help me do that work faster than a 4090, but it probably won't be world's apart. But the legitimate tax situation (all above board and vetted with my accountant), means I can sell now for £1,200 and with my adjusted spend being ~£1,600 later, I would get an upgrade for about £400.