Do you have to use the striker block in the kits? I’m mainly looking for a better trigger shoe and am fine with the feel of the standard striker block. I would use the included sear and actuator.

Not 100% sure, but probably not.
Though every version of the M&P 1.0 and 2.0 I've owned has felt better with with the APEX USB in it.

Swapping the striker block would require me to pull my optic and remove the retaining plate. Too much hassle when my striker block is pretty “polished” from use.

Should be fine.  The Apex striker block does the same job as factory - but the outside/bottom edge is more rounded and polished to help reduce and smooth out the trigger pull.

Sometimes, one some pistols, installation of the Apex striker block can result in the trigger bar not fully raising the Apex striker up out of the way to clear the striker so it can striker the primer hard enough to ignite the primer.  Sometimes there is a timing issue and the trigger bar can be adjusted to compensate for that.

Using the factory striker block should cause any problems if you're happy with the way the trigger pull feels.

I’ve done two swaps, and each time the striker block swap itivuded a significant and noticeable improvement.

I’d recommend doing the swap.  If I could only bake I’ve change, it would be the Apex striker block.  While any sort will polish/wear in over time, the Apex Striker Block has improved geometry that  never going to happen with use.