I thought certain common usb configurations could handle up to 15 watts. USB 2.0 I think only does a few, but I thought the next tier had some more capacity. The battery charging types would be more than plenty. I don’t quite understand usb though, take this with a grain of salt. Maybe they should do like a 6 or 12 volt power supply instead. It could also handle higher power lamps and you can pull many more amps thru that?

USB does not, Power Delivery does. But needs special handshake communication over the data lines (although the first 5V only revisions it meant certain combination of pullup/down or a resistive divider on the data pins).
 
Those are two different things, even when running on the same connector on the same device.

Only the USBC connector actually allows both (data plus high power) running at the same time, but you need again special handshake for it.
And many USBC won't give you anything at all before you signal the power supply back what you need (that is why many simple devices won't charge on USBC "high speed" chargers)...

To simplify things, we're going to get these small cheap 3w germicidal ballasts and make our own heating circuit still https://www.aliexpress.us/item/3256805308817130.html

These are high voltage low current ballasts for CCFL style cold cathode lamps, circa 5ma at circa 500-1000V. They plainly will not work with ~30V 170mA F4T5 tube, well okay, until you just want to kill a tube in a few hours by dimly running it in a glow discharge

A realistic solution with off the shelf parts may be getting a USB-C trigger board to get 12V with a reasonable power + some good step-up converter to get 150-300VDC depending on the ballast nominal voltage + off the shelf small T5 ballst (or integrated CFL guts), most will run on DC.

USB C power delivery triggers are cheap and easy to get. Then you have 20w to work with.

Assuming you have a USB power source capable of delivering this, of course

Well, a properly EU imported USB power supply that can deliver 20w costs 12 euro here, in a non-discout shop even.

If you limit yourself to the most basic 5v 500mA standard, it's obviously very limited.