This is probably fine for simplicity, but note the existence of WINED3DPMISCCAPS_INDEPENDENTWRITEMASKS (and "NumSimultaneousRTs" more generally); this is not strictly a requirement for D3D 9.3.
Is it not? [1] at least states it is. For d3d9 we'd presumably want to set WINED3DPMISCCAPS_INDEPENDENTWRITEMASKS appropriately (which, granted, I don't have a patch for yet) but in terms of setting feature levels it seems right?
[1] https://learn.microsoft.com/en-us/windows/win32/direct3d11/overviews-direct3...