Factory Fuse Boxes?

Has anyone successfully used the factory fuse and relay boxes to power aftermarket accessories (2021)? I'm planning to install seat heaters and an air compressor, and I would much rather use the existing fuse/relay box instead of adding an external fuse box.
Here’s what I need:

• One 15A circuit or two 7.5A circuits (no relay) inside the cab
  
  
• One 30A circuit (w/ relay) in the engine bay

It dosen't make a differencw whether or not these are switched by ignition, the control circuits will be powered by tapping into the 12V outlet in the center console.

As far as I can tell:

• The interior fuse box (driver footwell) appears to be completely sealed and uses specialized connectors. It seems like, using it would require a very detailed wiring diagram and specific terminal pins. Fuse taps might be an option, but they would prevent the cover from fitting back on properly.
  
  
  
• The engine bay fuse/relay box looks more promising. There are quite a few open fuse and relay slots, but most are missing the metal contacts.
  
  
  
  
  I'm not sure:
  • Where to source the correct terminals/pins and relays,
    
    
  • If those relays are rated for 30 amps,
    
    
  • Or how to disassemble the fuse box to access the back side for wiring.

Has anyone gone down this road before? I'd really appreciate any advice, part sources, diagrams, or instructions you can share.

Thanks in advance!