the front brake is responsible for most of your braking power in any setup> a rear brake requires more energy and more force to fully stop a bike than a frond brake does. you could get a bigger cog to tension the chain more but you already have a 17 T cog on there if you go any higher you may end up spinning out quickly on downhills. any higher than 17t you may spin out at a really low speed. also chains tighten and stretch the longer you ride and depending on how often you oil your chain. you may end up switching to an 18T only to find out a month later that your chain has stretched a bit and is now just as slack as you started with. a chain tensioner is really your only solution unless you get a new frame