Starfish is amazing… Never be afraid to ask for help visualizing. 
It’s by far the easiest position there is. The missionary position kills the man’s back. In 28 years, you could count on your fingers the number of times my wife and I have done anything but woman on top.
easiest for the man, sure. I find missionary easiest for me and me on top to be decidedly more of an effort.
Sure- but I maintain woman on top is easier on woman’s back than man on top is on man’s back.
It has long been my understanding that it’s usually the man who initiates sex, and many women are outright uncomfortable about doing so; if she wants sex, your advice could easily just end up with both of them sexually frustrated waiting for the other person to start something. Instead of trying to guess like that, he should just ask.
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Easier on her back, sure. Rather harder on her knees, thighs, and glutes. Not that I’m complaining (well, except about the knees), it’ll give you the hindquarters of a jockey and it’s way more fun than squats at the gym.
I knew there was a reason. I’m doing it for my glutes!
Two things.
First, have you tried having her on her back on the edge if the bed with you standing? Depending on your height and the height of the bed, you can add pillows under her hips as needed. She’ll need to keep scooting out to the edge (it’ll feel a bit like the gyno office … scoot a bit more … a bit more …). The angle of penetration in this position can be quite nice, and it would allow you to support her legs a bit and find the angle that works best for you both right now.
Second, is her reluctance to be on top a matter of shyness or discomfort? I didn’t like that position for a long time because I was sort of squatting back on my heels. It stretched my hips too much, and it hurt. I don’t know why it took so long to figure it out, but I now put my feet forward and much fun ensues. Just something to consider.
So that’s what it’s called. I came to the thread to suggest it. We’ve been doing it that way for years. Called it “the slow, comfortable screw.”
I don’t understand. There’s an emotional issue here? It’s not just a question of anatomy and mechanics?
That’s called “The Lazy Dog”. 
That’s the name I made up for it. It’s always been my fav.
Have you guys considered going to an adult store that caters to married couples (they tend to be more woman-friendly, with less hardcore stuff). They sell things that can make sex play fun and easier. Have you tried a sex wedge? They are really good for taking the pressure of the spine and can tilt her into an angle where you won’t put too much pressure on her belly.
Vibrator play = great suggestion
Also, oral is cheap…and awesome. I would also not give her a 100% pass on girl on top. Lovingly encourage her to continue trying, even if you have to wait until after she has the kid. She’ll never get comfortable with it if she avoids it forever. Make it a funny thing, like a game. That will ease some of the performance anxiety. It’s just play. Enjoying each other’s bodies doesn’t always need to be a serious, soul-searching deal.
How you might encourage her to try woman-on-top is going to depend on her reasons for being uncomfortable with it. Have you asked her?
Some women wish to take a more submissive role in sex, so she may be on top wondering what the heck she’s supposed to do now. If that’s the case, you can help (and believe me, it’s pretty hot) by gripping her hips and guiding her. Gently and firmly push and pull her back and forth. If she still feels like you’re taking the active/teacher/guide role, it may feel less awkward for her.
One possibility that hasn’t been mentioned, that I’ve seen.
How would she feel about woman on top if she was wearing a shirt?
Maybe she just feels ‘exposed’ sitting upright totally nekkid and bucking around. And she may be extra self-conscious about her stomach right now.
“Enjoys doggy but gets tired very easily (height difference)”.
Have you considered an inclining pillow?
Maybe also keeping her bra on so her boobs don’t flail around. She might feel less naked that way.
Or course, for us men, that take a lot of the fun out of it, but you can eventually progress to that point where she’s topless and her boobs are flopping about, the way things ought to be.
That’s what I came to suggest. You don’t need any special pillow, just a nice, solid throw pillow or two that she can put under her knees to raise her up so she doesn’t need to arch. Putting one under her butt when you give her oral will ease the strain on your neck as well.
This is what worked really well for my wife and me when she was in the last stages.