Here's one coming in from left field. The Tisas PX-9 gen 3 models meet your specs aside from being a polymer frame. RMR cut (no mounting plate required), magwell, reversible mag release, 4.11" barrel, better than decent trigger, and it uses P226 18 and 20 round round mags! Longer threaded barrel models are available. If you spend more than $350 for the 4" barrel models, you got robbed! I have the 5.1"TB Nightstalker model and love it. I have a Holison 407c waiting to put on it.

The only mainstream gun that fits your criteria; striker fired, metal frame and optics ready

Is the P320 AXG.

There may be another but Im not famalier with one.