I think the wheel was too small, that’s why you have a hollow. Next time you grind a near wedge blade you may want to use the biggest wheel you can get your hands on, the smaller the wheel the more hollow you get… try a 12 inch wheel (though it may be too late for this blade).
In the old days they used slow spinning wheels (slow compared to modern machines) and moved the blade sideways across the face of the wheel so the grind would be even… but I am not sure of that’s possible with a belt sander… I have never used one. There may have been a chance of saving the etch… in cases like these I usually snap a photo before I start work just in case… but it’s gone now.
If you no longer have access to the grinder then you could hand sand with low grit paper and press the paper into the hollow with your finger while you sand from heel to toe, and avoid going near the edge, that may even it out the hollow a bit and you would have a reasonably good hollow ground razor… complete with Hamburg grind… and then work up the grits and polish if you like… but it’s a hell-of-lot of work.