I'd rather an A7 III than a rangefinder, an A7 IV if you can swing it. I'd never even consider shooting that lens on a small rangefinder style body.

Not to mention that at that focal distance the autofocus is going to miss a lot on effectively the same sensor as the a99. Maybe not by much, but half a millimeter and getting a sharp stick and fuzzy bird kinda sucks. For not that much more the a7c ii is a huge upgrade over the 6700. The a7cr is a lot of tech for the price too. Or the couple extra ounces for an evf isn’t nearly as big a deal as the gear peddlers make it out to be. Good luck.

That's a big lens on a small camera. I'd find one of the A6000 series cameras locally and test it out before you buy.  I don't know which lens adapters work with which APS-C bodies so check into that carefully, they're not all equal. Also, you probably know this already, but in case you don't, you'd be shooting an effective focal range of 225-1200 mm. Fun, but it's going to be a bitch to hold still when zoomed out!