I'm trying to build a remote relay that can support >1500W AC.
So long as you're not switching an inductive load, and a heater isn't, then look at a contactor.
A contactor is basically a relay on steriods.
Looks like you're only switching about 12.5A (@120V), or a bit over 6A (@240V), but you want to over rate the contacts a bit (maybe 20-50%) for longevity/reliability.
Now, in order to switch the contactor, you will need to connect a suitable relay to the Pi, and there are many boards/hats out there with relay ratings high enough to do this easily. Yes, you are basically switching one relay with another, but the relay connected to the Pi only has to switch the coil in the contactor. Maybe an amp for the contactor coil. It is the contactor that has the serious contacts to handle the current you are talking about and do the heavy lifting.
Also, you do not
want to get a multipole relay and connect the contacts in parallel. To the naked eye, the contacts connect at the same time, but they don't. The contact that touches first will take most of the current and wear the most. A contact used in this manner probably isn't up for the job by itself, or you wouldn't have connected them in parallel to begin with, therefore it is a problem waiting to happen.