A lot depends on how complex and wide a profile you want/need. Wide pieces are going to be very diffficult to bend, so are usually made by joining segmented pieces of wood in a rough arch and then cutting a smooth curve. Thinner decorative elements can then be bent to fill out the profile, or if you have a lot of money or patience and skill milled from solid wood. More wasteful of material would be to cut the arch from a piece of sheet goods, I'd probably use a cPVC product (Azek in the US) to avoid having to deal with a plywood edge. Smaller cross section material can be bent-- Azek is good for this -- and you could build up a header from multiple pieces.

All that said, there's good reason that many such features are done in plaster. Above the door they are unlikely to take much abuse. I'd think hard about using plaster for the curve above the door and wood for the straight sides.

If you're lucky you might find a prefab PVC arch that matches your door width and curve. Those work well when building from scratch, not so good for pre-existing openings. We also have products like "flextrim" that can be bent, but the profiles are relatively small and, IMO, pretty ugly.

You might consider a different trim option like the picture below that can be done easily in wood (the concept, not necessarily this particular execution)

Ooh I quite like that idea. Thank you!!! I’m not far from top of arch to cornice. I could merge them like the above to the crown. Thank you!