Would these Craftsman trim proportions work with a low ceiling?

That sounds like a reasonable starting point for a shorter room. A 1x4 head can still read as a little wider than 1x3 sides without crowding the ceiling. Before buying everything, tape up cardboard or hold sample boards in place so you can judge the whole opening, including any small cap pieces.

Try the baseboard-to-casing corner in the mock-up too. A slight inset can look tidy, but the transition should feel deliberate rather than like the pieces almost line up.

I’d be cautious about changing the side casing size just to make room for a larger head. The proportions matter, but so does keeping the door and window trim consistent throughout the house. If the wider header looks cramped above one door, a simpler head may be cleaner than stacking extra strips there.

Mock up one opening first, then use that same layout for the other openings unless there’s a clear reason to vary it.

For painted trim in a dry room, MDF is a practical option and can give you a smooth, consistent look. Finger-jointed wood is another paint-grade choice if you prefer working with wood. Check the actual board dimensions before settling the layout, since the sizes printed as 1x3 or 1x4 are nominal.

If you want a wider, more substantial casing without buying one broad piece, you can build it up from simpler flat trim and a narrow backband. That gives you some flexibility with the profile and width. I’d skip using crown or baseboard as casing, since their shapes are made for different locations and can look out of place around a door.