It does strike me as fast, perhaps because it makes sense to me that there be some release and tranquility after the immediately preceding “storm” movement. In more practical terms, I would remind you that Beethoven performances are typically somewhat slower than his metronome markings. So even a faithful 80 m.m. reading will strike audiences as brisk.

You have the seeds of a further argument right there in your question, though, when you say that the orchestra can’t quite handle your idea. There is so much repertory available that there is little reason to program anything that the orchestra can’t play well. And, generally, there is little reward for you in leading a poor execution no matter how wonderful your idea. But this thought runs even deeper: We all have great ideas and dream performances in our heads. A challenge in this profession is to conduct what’s actually happening on-stage. (And, let me be clear that this is a challenge for even the most experienced conductors.) The audience can’t hear what’s in my head and is not obliged to sort through the performance keeping what’s good and dismissing what doesn’t work—even in cases where it can parse the performance in such a manner.

I wish you well in this performance, but it sounds like you should hold this idea for a future performance, for experimentation on a synthesizer, or just for pleasurable reflection during life’s tedious moments. You also might consider experimenting with simpler symphonies; Beethoven’s 7th and 5th are far simpler technically than the 6th. Let us know how this performance goes.