If it was me, I think I'd lean towards just having a smaller 12v LiFePO4 battery inside the box, though my original notion leaned a lot more heavily on solar than you planned to do, and I'm still working out some kinks.

Since there are a few different ways to do all this, I think I'll post options in general terms and can I'll point out that there are multiple specific product options that you could pick from, and I'm just citing examples I'm able to easily find. Cheapie Aliexpress equivalents abound for lower cost, I'm sure.

The thought I was working on was basically to get something like a Bioenno battery of whatever capacity you find suitable (3Ah at 12v is comparable to a "10,000mAh" lithium pack, ish), then use the PowerPoles on there to connect to a power distribution block/bus bars or similar. Use the power distribution to connect to everything else: 12v input for a USB hub, your 12v to 5v adapter for the Pi (I know that's not enough amperage, but it was a conveniently available example), or just a USB-C charger, or whatever other devices you think would work. If it was me, I'd definitely put one set of PowerPoles on the distribution block on a lead going to a panel mount PowerPole, it'll make the modular connections and charging easier.

Bioenno batteries have a barrel jack for charging and you can get an AC power brick to charge them. You can also get a solar charge controller for the system. The Bioenno battery has it's own built in protection circuitry so you don't have to worry about low voltage cutoffs or anything like that. Moving one or two of the PowerPoles from the distribution box to a panel mount would let you access the battery from outside the box for charging it, or another output.

Depending on size constraints, some of these components could be located inside or outside your main enclosure (for example, a solar controller could be inside the box with a "solar input" on the outside, or alternately the charge controller could be under the solar panel, and you could have a lead from the controller to your box that's just on the panel mounted PowerPoles direct to the battery to save space inside.)

It'd be easy enough to put the whole power distribution block "downstream" from a suitably large switch, or switch/relay, or alternately just have switches on individual components, or leave it all just on all the time.

It looks like what you'd want in order to charge this from your larger 12V battery is a "DC to DC Charger" which is a thing I learned exists just today! There seem to be lots of options out there. Same as the solar, locate the DC/DC charger wherever there's room, and have a lead going between the two batteries.

In theory, if both batteries are close to the same voltage you could connect them directly together (for example, have one of the sets of PowerPoles on the power distribution run straight to the other battery via one of the panel mount PowerPoles, a cable of whatever length, and then screw terminals or whatever the other battery has). The batteries would effectively be in parallel at that point and you'd drain them both evenly. This is... terrifying, a bit, though. If the batteries differ greatly in voltage (if one is fully charged and the other is near dead, for example) you might get a ton of current on that connection and damage one battery, or the other, or both, so... I'd stick with the DC-DC charger.

If you want me to clarify what this might look like in a diagram or add more detail, let me know (though I'm gonna post some other options too, so you'll have a few options we can elaborate on).

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Edit: Don't forget to make sure everything has fuses!