Your commitment to low memory usage is quite apt in the current memory crisis.

Looks like USB PD 3.1 supports 28V

If you need a bigger fan then Geek Air sells a 12 inch camping fan. It has a battery but crucially it has a DC barrel jack for charging. I personally own one so I can tell you the charging brick has a 27V 1A output suspect a 28V input would work just fine. Or you could make your own charging circuit with a buck converter or something.

Its a solid fan. Not sure what your needs are but it’s another option.

I bought it on the jungle website but the model number is CF1SEB if you want to buy it somewhere else.

If USB C is an option then 12V USB C PD cables are a thing.

The most overbuilt option is probably this: https://www.noctua.at/en/products/nv-fs1

You can mix and match components because at it’s heart it’s just a PC fan in a custom frame.