Just purchasing a 9 channel rx will not solve your problem as the problem is being caused by inadequacies of the tx. In fact, even if you have the 9C tx, it won't do dual elevator servos with your 7 channel rx as the default mix for that uses channels #2 and #8, so the 7 channel rx won't play with the 9C's dual elevator programming. Then you get into the latency issue because of that channel spread...[]

Some people choose reversing Y-harnesses to take care of the problem, but those have their own set of issues and I would not use them on my models. If you really want to retain the 7 channel tx, the best solution is to use a Matchbox (JR or Futaba calls it an MSA-10) as they have independant servo adjustments for reversing, centering and travel limits.

But those are about $60 a pop...

If you in any way anticipate continuing on with bigger models that use dual elevator servos, consdering your options, I'd suggest that it's time to start considering a new radio.