How much is too much? TermStore supports 1 mil. items with 30K per term set (2k for managed navigation). I assume it's the depth that is your problem, as the TermStore will only go 7 levels.
If that is the case (the depth) - I highly recommend you re-visit your navigation hierarchy. Going more than 3 levels deep becomes a usability nightmare.
You can switch from Managed navigation back to static and just populate per site, or you can create a custom navigation provider and replace the quick launch on your page. You then use a custom sitemap provider that points at your data source. That takes a bit more work though of course.
MSDN Refrence for custom navigation: