They make nice decorative blocks primarily to mount external lighting sconces to uneven surfaces. That might be more attractive to you.

What's the outside wall made of? Can you cut a hole in it for the box to extend into?

They make nice decorative blocks primarily to mount external lighting sconces to uneven surfaces.

Sounds like the ticket! Google not bringing up the right stuff... do you have more precise nomenclature... or links?

This Old House shows a sturdy FOAM block for evening out clapboard siding, but... not clear that can take a Decora switch cover.... hmmm... definitely on the right NON-metallic track. Wiremold Corp.?

What's the outside wall made of? Can you cut a hole in it for the box to extend into?

Nay way. I misspoke. It is not a (hollow) concrete block hurricane bunker, but rather an 8" thick solid concrete (tilt-up) BOMB SHELTER... box is mounted on pre-cast door frame... no jackhammers shall attack the structural perfection. ;')

"Got the ol' SHALLOW-box-on-exterior-wall problem... CBS construction, 1" furring strips, box is barely deep enough for std. single-pole switches."

And likely has exceeded allowable box fill also.

4x4 boxes with a plaster ring are about as shallow as you can go and still meet box fill rules.

You actually apply plaster to the plaster ring where it covers the face of the box.

Plaster I'll hack back from the mud ring and touch-up, reinforced concrete--no, lol.

RE: boxfill... IIRC, though they potentially generate some heat, dimmers count the same as toggle switches in that regard (# of devices), yes/no?

Also, the solution I'm looking for would ADD cubes, would it not?

PS: We're talking metal j-boxes and EMT conduit strapped to concrete walls behind old school (mid-50s) diamond lathe and plaster... is there a more fire-resistant type of residential construction? If the hack-job illegalities I've uncovered haven't even scorched a light switch, I doubt new dimmers, correctly installed, will create a hazard. NOT adding any conductors, so should be able to meet boxfill.

Devices all count the same for fill (despite dimmers being larger than switches).

Just because it is installed does not mean it is correct.

Having to dig into poured concrete for box depth is not the worst thing in the world.

Drill some holes and chip out between them.

The EMT makes rework harder, but not impossible (slip connectors are your friend).

Fit can also be affected by the brand and style of GFCI device.
Some are smaller and have better terminal locations for minimum allowed box size.

A device with side screws can touch the back of the box you just need to dress how the wires come off the screws.
Pinching wires between the device and the box (especially a metal box) can cause shorts, sometimes nasty ones that are hard to diagnose since they are not solid enough shorts to trip the breaker all the time.

The buzzing and smell may be the only clues.

I don't know if the appearance will work for you, but Wiremold makes open-back boxes intended to originate a raceway run from a recessed box. One of these might give you the depth you need. Paint it to match the wall.