As far as I can tell, Kizer Elements have the widest groove with the wheel sets packed quite close.
The Fluids also looks to have a wide groove.

I'm currently on a semi-rocked setup with two 59mm M1's and two 56/57mm random wheels on FLT-2 frames.
For a flat setup, I'd prefer using those wheel sizes, which the Elements frame will take.

Any other frames I should consider?

Yes, I know a setup with rockers will be better, which is why I'll want the wide groove.
I'm having a major time adjusting to a two-wheel setup, because I also ride freeskates..
I could make the transition later when being more rehearsed.