I am 100% with you - I bought my mummy bag at a garage sale, and I’m taller than my bag, so it often slips off of me in the night haha. To counter, I sleep on my side and pull me knees in a bit and it doesn’t move as much
The two major benefits I’ve ever seen in mummy bags have to do with 1) cold weather camping…a close tapered fit eliminates cold air space and 2) mummy bags have less material and size so they pack smaller for backpacking. So if you have no direct need for either points…use a rectangular bag or quilt.
If I’m not sleeping below 20 degrees, my mummy bag is always unzipped and utilized more like a quilt.
mummy bags are supposed to move with you - unlike traditional bags you can roll around in. When you roll over in a mummy bag - it can turn as you do. Have you considered trying a hybrid bag? I have heard great things about The North Face Eco Trail model - I think next on my “to buy” list (also available in an extra long version) https://www.thenorthface.com/shop/eco-trail-synthetic-20-nf0a3s76
Check out the Zenbivy sleep systems. I bought one a couple of years ago. I love it so much I sold my beloved Marmot mummy bag. I usually sleep in a hammock when I camp, but the system is primarily designed for ground sleepers.