Have used Feit (from Costco) with great success. Paid $19.99 for a three-pack with three -way dimmers. Of course they work as ‘singular’ units as well. Better built than KASA.

Anchoring nut? You might be able to reuse from old non-smart TRVs. As they’ve been around from the 70s, there should be an ample supply (somewhere).

Don’t overthink it. Place a smart switch (dimmer/three or four way) in the location that has constant power (and a neutral of course). The other locations just regular three way switches, nothing ‘smart’ is needed there. Use your favorite home automation ’service’ to control everything. In my case Alexa. I have switches I haven’t physically touched in months.

If desired, you can still physically turn on/off from any (three way) location, just set to whatever dimmer level the main happens to be at. If you want to dimming level either touch the main, or call out something like; ‘Alexa dim hallway to 80%’