When I’ve had a cold like this in the past, as a dedicated front/sleeper (gave that up weeks ago, alas!), I’ve always made myself a bit of a pillow nest. I make the pillows into a v-shape so that they support me on my side, including down to my waist.
I start with a pillow with one end under where my head will be, and the other in front of me, at an angle. The second starts at the same place, but is to support my body - it’s at about 80 degrees to the first, and I want it to go down to nearly my waist. The third is the same as the first, and the fourth is like the second, but optimised to support my head and get out of the way of my shoulder, while not being in the way of my face, because any obstruction makes breathing harder at that point!
It takes a bit of arranging, but I’ve always been able to get comfortable that way with a bit of careful propping up, making sure my torso is held roughly in a straight line. For pregnancy, you might need to get a bit inventive with pillow positioning, particularly around your bump, and you might find that your normal pregnancy pillow, if you have one, has a part to play somewhere.
If you do decide to keep sleeping on your back, is there any way you could use loads of cushions to take your whole body up - like a wedge? The more upright you can get (while still totally reclining), the better it is for blood flow to the womb, I believe, and the straighter you can keep your neck, the less uncomfortable you will be.
I hope that you manage to find a comfortable sleeping position.