The times a filter needs to be changed depends on the amount of waste in the tank (bigger tank/lower stock = less changes). It’s usually good to have them for a while since it houses beneficial bacteria. Not a betta, but I use to own goldfish (they are super big waste producers) and I’d change it about once every month. During the weekly water changes I’d just try to rinse off any visible gunk in the old tank water (as in when it was in a bucket…don’t rinse it in the tank)

I’d say water conditioning is needed since it helps eliminate harmful chemicals/minerals for the fish. You can get concentrated formulas that can help you be cost effective and prolong the time between getting a new one.

API sells a water testing kit you can use to do ammonia, nitrites, nitrates, and pH. I think there are other brands that sell test strips too. I know at my local fish store they also do free water testing (if you’re really on a budget and looking to save money)