I have an outdoor solar powered (200W panel) Raspberry Pi setup that was running off some (non deep cycle) car batteries that have now died, so I'm now looking for a new power storage option.

The easy solution would be to get a 12V AGM battery but those are quite expensive ... Recently I saw a sale for a 30Ah (30x3.7=111Wh) USB-C PD power bank for only 30 AUD, which is a lot cheaper than buying individual 18650 cells not to mention the BMS and output circuitry (used 2Ah cells here are around 2AUD). The loads are Pi peripherals (5-10W) USB hub (5W-ish). This power bank does support pass-through (charge while running output), and I'm quite sure the capacity rating is legit (official store).

My idea was to get one of those cheap adjustable MPPT modules (like this) and set it to 9V, which the power bank accepts (up to 2A or 18W). Say the average draw is 10W I should have 8W leftover to charge the batteries - obviously not ideal but I'm thinking I could scale this up easily with multiple banks charging simultaneously (perhaps run the USB hub off a different one).

I guess my question is, does this setup seem reasonable (if not overcomplicated?) Even with two banks I'm looking at 200 Wh which seems to be a very good amount of storage for the price. Perhaps there's a better option? And as a side note anyone know of any power banks that charge faster than 18W?

Thanks (and sorry for the wall of text).