Schematics model are not exact, but that's a lot. Maybe your Momentum setup is still bad.
Did you implement the little extra 50 ohm feedline, to make sure your port width at the feed is correct?
Also, did you include metal thickness is Momentum (thick metal model instead of infinitely thin?
And finally, high mesh density is the key to accurate EM simulations, so make sure to enable edge mesh.
If you are able to reach reasonable agreement between schematic model and EM, you can optimize fast at schematic level. Otherwise, you will need to use EM-based optimization. My appnote might give some hints how to create a parametrized subcircuit for the emModel workflow.