The uConsole has enough space in the back to fit an extra 18500 (as opposed to 18650) Li-ion battery. The maximum capacity of these seem to be ~2000mAh, which is what I got. You can get those that have pre-soldered wires and you’ll only have to attach the other end.
Yes, it’s not ideal to use 3 batteries with different capacities. But all 3 are in parallel – the axp228 PMIC doesn’t really care about the number of batteries anyway. Even with just 2, you fully rely on natural balancing that happens due to the fact that they are connected in parallel.
With a different battery configuration you’ll also need to reconfigure your device tree and re-calibrate the PMIC to get accurate reporting, something like what I did with the Arch kernel: Arch Linux ARM for uConsole w/ RPi CM5 - #42 by PeterCxy
(None of the battery mods solve the charging speed issue. Charging current is limited by the PMIC and I don’t think you can fix that without getting a different PMIC.)