Separate panel for house in garage - how much capacity do I reasonably have left?
Go 60A on the Tesla charger.

You'll thank me later.  

Your main panel has the capacity for it, so let's just bite the bullet and do it.  

You can serve it out of the main panel without any trouble.  6/2 cable and you're done.  

If you want, instead, you could replace that 90A cable with a 150A cable.  At that point you can put the 60A charger in the big subpanel.  I presume your awesome 40 space sub has 200A busing.  However I think that will net out to a lot more money.