Can I ask why you'd want to be level 6 at that time?
I know in the US there are options to compete for adults, so I think if you want to you can keep growing for years to come. I started gymnastics at 24, and in my group there are people who are nearly 60 who can do skills most kids can't. So it's not like your body just stops being capable on your 18th birthday or something.
Of course you don't have to do gymnastics that long if you don't want to, but I'm just wondering why you're in such a rush?
I agree, talk to your coach, but maybe it'd be a good idea to ask stuff like how long would it take you to get to level 6, and where could you be by the time you're 16. Also let them know if you're willing to do extra conditioning at home. I expect that could really help you with bars.
Also, I'm curious, when you say on bars nothing: can you do a pullover? A back or front hip circle? A cast, or a squat on? Any dismount? Tap swings on the high bar? If the answer to any of those is yes then it's not nothing, don't talk yourself down too much

. There are plenty of people who can't do any of those. If you cant do any of those in 2 years of excel silver, then I would worry a little bit about the coaching, and I'm here to give any tips or conditioning that may help.