Refuelling problem

It sounds like the fuel level sensor might need to be recalculated. That's guessing that the tank is not empty or nearly empty when it says it is, but is full when it says it is.

There's a chicken dance involved. Here are the perfectly intuitive steps the Toyota engineers came up with:
Do on level surface
If car is on, turn it off
Turn it on
Put it on trip A
Turn it off
Keep foot off the brake
Press & hold trip button
Press power button twice
Release & press trip 3 time and hold on the 3rd
Wait till odo displays series of six letters & numbers
Release trip & press & hold it again
ODO will change to 5 letters & numbers
3rd digit quickly change to a 1
Release trip button

 

the evap cannister may be contaminated.

also, your bladder may be acting up

 

The way this is described, it seems like the internal volume really has been reduced to around 3 gallons; that's what would explain that not only does the filling nozzle shut off at that point, but the car's fuel gauge really does run the gamut from empty to full with that amount of fuel being added.

Because Gen 2 has the tank with a bladder inside, one way to reduce its effective volume like that is for the bladder to be unable to expand the way it is meant to.

In order for the bladder to expand as it is filled, and contract again as it is used up, the fresh air in the tank outside the bladder (blue in this drawing) has to be able to freely exit the tank as the bladder expands, and reenter the tank as the bladder contracts. If that air is trapped in the tank outside the bladder, then the bladder is effectively stuck at one size. So the valves and other components shown on the blue air paths here are things I would be looking at early in the investigation.

 

As some of the components that might be at fault aren't part of the tank, jumping to tank replacement before properly diagnosing the issue could turn out to be an expensive no-op.

While evap and ORVR systems are among the trickier systems mechanics train to diagnose, mechanics do train to diagnose them. Somewhere, you ought to be able to find someone who can. Or, poke around the technical library;

Toyota Service Information and Where To Find It | PriusChat

and bone up on doing it yourself.

People mostly sign in there for access to the repair manual (which you should definitely have). But once you are there, don't overlook the Technical Library tab. In there, you will find a bunch of other stuff, including the course materials for the "University of Toyota" technician training courses. The ones for evap and ORVR diagnosis will give you a much better grounding in that than you'll get just from the repair manual.

 

If the chicken dance didn't work, it's probably something to do with the bladder or the evap system as already mentioned. Seem to me that it's more likely the bladder for the reasons @ChapmanF detailed. Like him, I prefer to make sure as possible of the cause before possibly buying something that won't fix it.