I'm new to betta keeping myself, but I think I can help with some of your questions. Your food variety sounds great. Just be careful not to overfeed because they are prone to constipation, which can kill them pretty fast. Some people recommend not feeding one day a week and the day after that, only feed them the inside of a cooked pea broken into small bits. That is supposed to alleviate the constipation problem.

I have a top filter in my tank. It causes very little water disturbance and doesn't upset the fish. It does require electricity to run, but don't you have to plug in your heater? Maybe you could add an extension cord for it. Water changes should be enough, but I prefer the filter so I know the tank will be kept clean. I change about 20% of the water each week.

The gravel doesn't matter to the betta because they don't spend much time on the bottom. They do love plants, though, and the gravel anchors the plants. I use small, smooth river stone, but whatever you like will be fine with the betta. They love to swim through small spaces and the plants give them places to do that. It also makes them feel more secure to have hiding places. I also made mine a tunnel with a short, curved piece of pvc pipe that I glued river stone to using aquarium caulk. He enjoys swimming through it. They are very curious and like having interesting things to explore, even though they may flare at new things at first.