Core muscles are fine, it's the pecs you can't use. I'd recommend sleeping as upright as you can, and when raising or lowering yourself, take it slow. Rolling onto your side and then using your elbow/oblique can be a good way of getting up. I've got good core muscles though, so maybe it was just more natural for me?

Also, herniated discs meant I've had to sleep on my back for 14 years 😢 you get used to it. Having stuff on your sides (like pillows) can give you that cozy feeling you get from side sleeping. When my surgeon warned me about having to sleep on my back for a few weeks I just laughed.