Ok, so to recap . . going to have to draw this out. You have an extra conductor which is probably sending an additional hot down the line. Just have to figure out which one that is.
Please double check this:
Black is connected to the switch’s brass terminal
White is connected to the switch’s brass terminal
Red is connected to the switch’s black terminal
Romex 1 - 3-wire:
Black is connected to Bundle H with the Reds
White is connected to Bundle N with the Whites
Red is connected to switch’s black terminal
Romex 2 - 3-wire
Black is connected to switch’s brass terminal
White is connected to switch’s brass terminal
Red is connected to Bundle H with the Reds
So at this point my best guess, pending some confirmations, is that your power w/neutral starts in Box 2, with the Line and Load in the same box. This means that the switch is powered from this box. It also means that the Romex to the light is in this box too. If that’s correct, this is where the Inovelli will go.
The thing that is confusing is that in a Line/Load in the same box, typically you will have 2 “outgoing” Romexs, a 2-wire going to the light and a 3-wire going to the other switch. In your case, you have two “outgoing” 3-wires. That’s not all that unusual, as the third conductor to the light can be used to send a constant hot downstream. It’s just a matter of understanding the original electrician’s logic.
In the mean time, a voltage measurement is in order. Do you have a meter? In Box 2, pop the wire nut off bundle H which has a red and a black from this switch (right?). Test for voltage between that bundle and the bare copper ground (or that bundle of whites if you want). I would expect a constant 120V. Do this test with BOTH switches in different positions so that you can confirm that the red/black bundle is ALWAYS 120V regardless of any switch position. I think that’s going to be the case, but please confirm.