how to improve tank for next betta

I had a betta back in may for almost a month. he was happy and very active after the first few days. so, almost a month in, my parents bought me 6 tetras - 3 neons and 3 glowlights - because they read that they were good tankmates for bettas. i was very nervous at first but they seemed to get along fine for the first day. on day 2, 2 of the neons had died, on day 3 a glowlights died and on day 4 another neon and my betta died. we checked all the water parameters and they were absolutely fine, and the betta seemed fine until the day he died. the people we bought the tetras from said it must have been stress even though we didnt know why that would kill the betta.

I'm planning on getting a new betta in august and want to make sure he lives the best life. there are still 2 glowlight tetras left. we expected them to die along with the rest but they've stuck it out for almost a month more. I do not want to buy more tetras as I've now been told theyre very fragile and I know I won't be able to handle it if they all die again. I have two plants but one of them has leaves that keep falling out so I think I might stop buying that kind (I don't know what it is since they don't name the plants at the shop). so what things can help keep a betta healthy and happy? im not so concerned about getting him tankmates anymore since it was disastrous last time, but I want to know if there's any little things I could get to entertain him or make his life better.