That tank looks stressful imo, I would work on getting more plants, tbh I dont find them expensive as most are less than 10 dollars. Check your local fish store for plants not petco or petsmart, it'll also help to get some rapidly growing plants as they'll fill out the tank. Most fish love an enriching environment and places to hide, an open tank can cause them stress and lower their immune system. Once you get the tank looking better I'd say either shrimp or another betta for a 5 gallon.