I know that DAK grips won't work on the DA/SA.
What about the reverse? E2, slim grips from other makers, etc.

Yes. Almost all grips should. Several DAK guns had grips with the de-cock lever cutout.

I have had multiple DAK pistols and swapped grips around with never an issue, both SIG factory and aftermarket grips.

Thank you, SM. Much appreciated.

Houge makes black checkered aluminum grips specifically for DAK models.

(they make other DAK grips, but that is the only style I use.)

They are like the DA/SA grips without the gap or cutout for the decocker.

I've had mine for many years with no issues to speak of.

Again, much thanks.

There are some external differences in the grip panel designs, a decocker groove, etc. From that perspective, the DAK doesn't care whether there's a decocker groove there or not.

But there are internal differences inside the grip panels, and as a result not all DAKs accept all grips. Found this out myself a while back.

First thing I did with my then-new P226 DAK was to put on P220 Elite grips, very pretty.

And - hey-presto - the trigger wouldn't reset after a shot. Turns out the DAK grips hollow out space for a trigger bar geometry that is different, and the standard grips don't. As a result, the standard grip interfered with the trigger bar motion and wouldn't let it move back after a shot.

This took a while to diagnose, and the fix was to get the Dremel out and remove some wood from the inside of a grip panel to create the space for the trigger bar to reset. It was not hard but took a bit of thinking to figure out where to remove the material.

Sigfreund or some of our DAK experts may be able to tell when/if the DAK design ever changed so that ordinary grip panels allow the gun to function - can't speak to that as all my DAKs are older vintage.