Glad to have you with us. As you have found, you can get plenty of enjoyment out of cheap bikes - keep riding them. Go to a proper bike shop with your bikes and ask them to check them over and adjust, because bad assembly is the most common problem. In particular ask them to check the spoke tension in the wheels. Tell them you are thinking of buying better bikes, and they will be glad to show and advise. The more you ride, the more you will know what you want.