Relay to Fusebox amperage

I'm running a 4awg wire to the bed of my truck with the intention of building it out similar to most overlanding builds. I'm doing 4awg because of the distance, and some of the items I want to have power for in the back (air compressor, radio, fridge, USB outlets, and LEDs). Both the air compressor and radio I will need 25 amp fuses, fridge 10 amps, while the remaining I suspect to be less than 10 amp fuses.

The Fuse box/relay I will be using is the following:

$27 True Mods 12V DC Auto Waterproof Fuse 5-Pin Relay Box Block [6 ATC/ATO Fusible Holder] 6 Bosch Style Universal Relay Block Holder for Automotive Vehicles Cars Marine Boat Offroad Jeep Light Equipment

https://www.amazon.com/Waterproof-Included-Universal-Automotive-Vehicles/dp/B08KHVHLL5

I want to have a master switch in the bed of the truck so that I may shut off all power to the fusebox/relay when I'm not using it. The master switch will need its own relay and fuse/breaker. What current should I get the relay for? I will be tapping into a breaker in the engine compartment rated at 60 amps, so is it necessary to have an additional fuse/breaker right before the master relay in the bed (I assume so to prevent fire between the bed and the engine)?

Not sure how ohms law applies here. I believe the fuse box is technically in parallel, but regardless I would just add up the maximum current load and that would make my relay more or less 60 amps assuming I would not be running the larger components at the same time.