Native switch/relay combo to support 2 to 3KW heater

Okay I think this is the answer :

along with this for manual control :

Sure, that should work if you only have one heater but I have 6 so it would be way too expensive.

Essentially you can - and here: SHOULD - build any “big”, safe relay connected in series to any OH-controlled low-power switch/relay (RPi GPIO, SonosSOnOff, ZWave actuator, whatever).
But high voltage high amperage is dangerous stuff.
You shouldn’t want to take advice from some part-time home automation software hobbyists
(like I am, too).
I hope noone feels offended by that comment of mine, I’m not saying it’s wrong what is written.
But speaking as the moderator I am, how to best build dangerous stuff is somewhat out of scope of this forum, you’re better off with getting advice from some site focused on electrics.

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Totally agree.
That’s what I did. I had numerous conversations with my electrician to come up with an appropriate (safe) design including double relays and circuit breakers to isolate all of the high voltage, high current from the OH equipment.
We also had clear separation of duties; I provided 12V as ‘output’ and he used it as his ‘input’ to switch the high current circuits.

Luckily I have an electrician in the family