Sounds to me like you are providing great care and for the tank size- that is fine too IMO/E. Don't feel bad or let others make you feel guilty.
IMO/E-1-2gal tanks unfiltered are great to use for long term keeping of the Long fin males. People often forget that this is a man-made/domesticated fish and after years of selective breeding-They have abnormally long-often delicate, heavy fins that they have to carry around. Often, with some, not all-of the long fin males-the water movement and even too much space can cause fin issues and added stress-both of which can cause compromised immune response.
What I recommend for water changes on 2.5gal unfiltered tanks-without live plants-Twice weekly...1-50% water only and 1-90-100%.
Your food is fine, however, when you can-I would get another fish food to add to what you have to provide a more varied diet. With feeding-sometimes we have go through a bit of trial and error-especially on the amounts since fish food can vary so much in size. It sounds like you are doing a good job, however, feeding small amounts twice a day might be better.
Your temp is fine too-all I use in my 1gal-5gal tanks-are the preset type heaters and they work fine. The temp can vary 5-10 degrees more or less based on room temp-but my adjustable heaters can vary 5 degrees more or less too. A healthy Betta can tolerate the gradual temp changes that vary 10-20 degrees that are often related to light-on/off, different levels within the tank...etc......It those sudden extreme temp changes that can be stressful.