what is a good wall mount for 85 inch tv?

800 x 400 is a big mount. You can these on amazon for about $50 (sometimes as low as $35) and are as solid as the ones that sell in-store for higher prices. If you want something fancy like a 180 degree side tilt, that will cost more.

Here is a standard wall mount @ $45 that should work, it's rated for twice the weight of your set (double check specs to be safe)
https://www.amazon.ca/Mount-Monitor-Art ... B00A7WDYYU

Personally, I like the tripod mounts but they cost more, unless you build your own. And you may prefer a wall mount anyway.
Here is a link to the 65" one just to give you an idea of how they look. They do have them for larger sets but you'd have search for it as I
don't have that one bookmarked.

I just got this from PrimeCable: https://www.primecables.ca/p-347462-cab ... #sku348142

Very strong, rated up to 200lb, most among all that I checked. It's not very flush to wall (~6"), as it's fixed but tilt-able. Can't have tilt unless there's some clearance at the back.
The best thing is the wall plate is very wide at 44", so you can likely attach it across 3 studs. This probably important for you with 800x400 needed.

Thought about getting full motion mount... but really don't think should move a 100+lb 85" tv around so went with fixed instead.