I'm a bit lost but it sounds like you just need the drywall to cooperated following the curve of the arch. One answer is, it doesn't need to be perfect. The flex corners on each side will act as guides for the mud and give you a perfect finish. I'm not a pro with the various compounds, but some of the quick set options produce a harder surface.
If you want a piece of wood up their first, using a radial arm saw or circular saw, you can cur a grove every 1/4" 80% of the way through. Soak that in water and it will bend.
OR, simply buy the flex board designed to bend around corners. I've always done it the hard way, but if a job came up in my retirement, the flex board is the way I would go.
Others here probably have a brand name for it.
Bud
If you want a piece of wood up their first, using a radial arm saw or circular saw, you can cur a grove every 1/4" 80% of the way through. Soak that in water and it will bend.
OR, simply buy the flex board designed to bend around corners. I've always done it the hard way, but if a job came up in my retirement, the flex board is the way I would go.
Others here probably have a brand name for it.
Bud
