To the OP:
Good on you for seeking the correct care for your fish.
2.5 gallons is okay, if that's all you can find/afford for now. The important thing is to take appropriate measures for the size of the tank to keep the water absolutely free of ammonia (the smaller the tank, the more frequent the changes - and the more stress on your fish, but some fish cope well with that, others do not..).
I have my little VT girl in a massively planted, filtered 3.5 gallon with 5 shrimp and she is as happy as can be, and super healthy with it. I am strict
with my water tests and 2 x 30% changes - used to be more, but with all the plants in there this schedule is fine for keeping the water pristine.
I have another little guy with some long term health issues in a barebottom 1.5 gallon, and while I feel kind of bad for him it's the best solution I've found to give him some relief from his various problems. I change his water 3x 100% per week, as any ammonia level large enough to show up on the test makes him very ill. And that's what it takes to keep his water clean, and him as healthy as I can make him.
My other guy is a huge plakat, who is soon to own a 10 gallon NPT that is heavily, heavily planted and filtered with a sponge filter as well. I feel this is the best environment for him.
I don't mean to carp on about myself, that was all just to illustrate that different fish can thrive in different environments.. not every betta needs a 10 gallon tank to thrive and have a happy, healthy life. And some will suffer horribly in a 2.5..
I think you'll work it all out. If she needs an upgrade, I have no doubt you'll eventually get her one. Because you seem like a nice and responsible fish owner, to me. :)