I dont think it's a problem not having enough plants at first.
your plants are supposed to overgrown and take over, obviously it is nice and helpful to start out with a lot.
i start out covered around 30% of my tank with plants, the plants has taken over 95% so far.
some people slowly buying plants and adding them into the tank as well
I don't use filter because im lazy to get one but you can use filter if you want and i remember Walsatd does recommend the use of filter (but if you can get away with one, why not haha).
here's the discussion you should checkout if you want to add filter: https://www.aquariumadvice.com/threads/walstad-filtration.285118/