Check for water bottle baffle instructions. Dead easy to make, cost nothing more than an empty water bottle and work fairly well. Most Bettas don't much care for current at all. I've kept mine in 2.5 G tanks with no filtration at all, so the only real movement is when the fish swim. But they have loads of plants, and I've been experimenting with simply topping up and not doing WCs.
So far, 3 weeks in, so good. No ammonia or nitrite, nitrate less than 10, and the fish seem content and I don't have to annoy them twice a week with water changes as I used to. If it continues to work out I hope to be able to reduce WCs to monthly or every six weeks or so
In a 5G, you could likely do this also and not change water very often at all. I use DI for topups so the water doesn't keep getting harder with mineral buildup.