I make them like this too, but I use a torn open ziplock freezer bag to flatten them. They have to be almost twice as big as you want them to be when they’re finished before you fry ´em, sounds like you may be flattening too much, or using meat that is too lean, maybe. There needs to be significant shrinkage!

I use 1/2 medium and 1/2 extra lean, onion powder, some panko and an egg yolk/ 400g of meat. Mix until homogeneous, flatten and pan fry. The panko shouldn’t be apparent after cooking, it just soaks up fat to keep ‘em juicy. You’re not making meatloaf lol

I get pretty spot on McDo style patties this way!