Betta Fish: New Betta

Hello! A few days ago, I purchased a betta from my local pet store. I had a betta as a kid and we really didn’t do much in terms of setup, so I didn’t see a problem with buying the fish and tank setup in the same day. Upon coming home and doing some research, I see how wrong I was! I got a 3 gallon tank and filled it with 5lb of gravel, a few plastic hidey-holes, a few real plants, a hammock, and a piece of mopani wood, as well as conditioned, bottled water. I (now) know I should have let the tank cycle over the course of some weeks but seeing as I already had the fish, I decided he would have a better chance in the big tank than in his pet store cup and introduced him slowly by floating the cup, pouring in new water little by little, and allowing him to swim out of the cup after a couple hours. I’ve ordered him a heater and a water testing kit (neither were available locally). He has Top Fin brand food pellets as well as some dried blood worms, mysis, and daphnia for supplemental treat feeding. Day 1 was fine, day 2 he had some inversion issues (like swimming diagonally) which I worried was because he was too cold thus ordering the heater, but now on day 3 he is swimming horizontally again and seems relatively happy? He doesn’t eat his food immediately but has been eating it eventually! He swims around a lot and seems to be curious about his surroundings, and he also likes lounging between the two leaves of his hammock. He occasionally opens his mouth really wide but he doesn’t do it often and doesn’t seem distressed so I’m hoping it’s just a quirk and not a sign of something wrong? I know the 3 gallon tank is likely too small and will be upgrading in the future but don’t want to stress him back out again so soon after his initial move. Any advice for a well-intentioned but ignorant new fish keeper? Pic included for reference (light is only on so you all can see the set up clearly, I’ve been leaving it off because I read it can stress them out). Thanks!!