I have some thermo regulated valves that require 1.5v as well, so for those I decided to go with regular alcaline batteries. Those are still above 80% charge after 1 year of usage, that is why I decided not going with rechargeables for those trvs

But there are 1.5v rechargeable AA batteries out there.

You can try your luck and see if you can find hardware similar to yours already probed, that may help you decide.

In my experience, persistent usb systems are painfully slow depending on the combination of components. Is your usb drive fast enough (usb 2? 3?)? Is your usb port fast enough? Habe you tried different port on the computer? Booting from usb will most likely require the system to be loaded into memory, do you have enough for what you are trying to achieve?

Definitely a la IE