I would like to know if anybody has done something similar, and if the behavior is what I describe. 

Is it possible to achieve a better behavior, for example by switching the travelers? 

I will probably replace the dumb switch with a smart one, and I expect it will work better in that case. 

This is the way I wired it:

1. The line/load of the 210 is connected to line.

2. Both travelers are connected up between the 2 switches.

3. The dumb switch connects to the load. 
4. Neutral of the 210 is connected to the circuit breaker's (common) neutral.

5. Ground is common. 

Given this, the 210 works as expected, the led status is indicating the correct status, turns on and off as expected,

including via wifi.  The self-check does not flag any wiring errors. So far so good.  

Oddities: If the dumb switch is flipped, the smart switch does not seem to know about it. 

Say, I turn off the load via the dumb switch: the smart switch still thinks the load is on.

Both  the led and the Kasa app shows the load as "on". when in reality it is off. 

When the smart switch is then flipped either physically or via Kasa, then the smart switch syncs up with reality/

(If the  load was turned off via the dumb switch, when I push the smart one, it will turn the light on and turn off

the led status indicator.. etc. So as long as I only use the smart switch, it works as expected. 

If I operate the dumb swith, the smart switch status will be temporarily off, until I operate the smart switch again.  

I think this is actually a reasonable trade-off for saving some work and money, if  your use case here is just adding remote 

capability to a 3 way circuit.  The only real downside is, that when operating the switch remotely, (when you cannot see  if the 

load is really on or off) requires that you  toggle the smart switch remotely 2 times.   An example of this, is when you are away from

home and checking if you left some light on, an want to turn it off remotely. 

One you toggled the switch 2x, you can then trust the satus casa is showing (until someone flips tghe dumb switch again) .