But yes to piggy backing relays, provided it is understood that is tandem - not parallel.
EG to boost any relay output, have it control another relay.
EG - to control a 400A load, use a 500A relay which is energised by your 2A switch, or 10A MW728 low-voltage disconnect, or your 80A battery-isolator smart switch (ie, a boosted $20 MW728; almost).
(Parallel switching to handle loads (as opposed to redundancy) can be problematic - even dangerous. And redundancy without alarms or inspection is merely a delayed failure.)
why not go via DTMF tones? you can get devices that use a mobile network and all you do is put a sim card in and call it from any phone anywhere in the world. then when you push the keys they send a tone to the receiver in the car and each done does a different relay.
i am planning to have something similar in my car as an override