You'll hate it if anything derails. And you have sun blasting out of the tunnel , I think that what's "wrong", lol Does it have an access opening at the window at all to block off?
It's kinda tall too. By closing the nook off so much, it's really giving the area cramped look. In the photo at least.
The open nook shape and border work can give it the feel of a valley, plateau, or clearing for that sun to play in. Maybe keep things running in parallel lines with pavement, track, buildings, etc. and do a fast rising horizon line above the windowsills line of sight is all you need to take more advantage of the light that always pours in there. And honestly, tinting it, or using a shade with a moon on it are still all I can think of to fight it.
I'd leave it open, push the horizon up and back, and use the exaggerated visual length on the oval from that angle, for giving you a good head on or trailing view out of a short distance, and some "down the row" views on scenes.
Or build a hillside to one wall, and eliminate half the tunnel? I know you like the portal views and that would open it up more, but keep a view.
You move fast and the changes are hard to keep track of. You didn't do all this since that post did you? But yeah, that would've been fitting. The bottom retaining arches especially, as the sill blocks were well used to keep them looking shorter. A section of open arches in front of the nook to allow passage, and some viewing . Staggered arches? Opened then.closed?