Reddit mentions of Black Ops Defend'R BMX Stem, Black

I'm 6'1" and like a somewhat upright position on my commuter bike. I also have several MTB's.

It's my impression that you may need to do a little customizing with aftermarket parts to get exactly what you want.

When you say mostly pavement, and maybe gravel roads, that would normally put you in the category of cyclocross bikes, "gravel" bikes, commuter bikes, etc.

However, you also mentioned getting tires as "thick" as you can get them short of being actual "fat bike tires."

So let's start with a discussion of wheels and tires.

What is Mid-Fat or Plus?
There's a genre of mountain bike called mid-fat or "plus" tire size. They are typically either 27.5" X 3.0~ish, or 29" X 3.0~-ish. Watch this video to figure out what I mean:
https://www.bikemag.com/gear/mean-27-plus-29-plus-bikes/

These tires would do great on Chicago snow. However, these type of rims and tires would absolutely only fit on a mountain bike specifically designed for wide-tire spacing.

So, what is "cyclocross" or "gravel bikes."?
https://road.cc/content/buyers-guide/205556-cyclocross-bikes-v-graveladventure-bikes-whats-difference
They are road bikes (with the roadie look of curved drop handlebars) with wider-than-normal and grippier than normal tires. However, these tires are still narrower than any typical MTB. Although these tires would be great for wet conditions, bad roads, gravel roads.... I imagine they would not do much for snow.

https://www.jensonusa.com/Mountain-Bikes?WheelSize=27.5%22+%2b

One issue with new mountain bike design is everyone is moving towards a slanting top tube and lower standover, then using tall seatposts to get the bike fit correct for tall riders. A "dropper post" (adjustable on the fly) is great for trail riding when you want leg extension for flats and climbs, but a lower post for rocky/rooty descents (or jumping).

So......... if you were to get something like the Kona Unit X,
https://www.jensonusa.com/Kona-Unit-X-Bike-2018 , I would suggest first getting the longest seat post you can buy. Or maybe even a dropper post. I bought a 170mm one recently.

Next you said you want an upright position. For that, you'll want taller handlebars. You could just get an angle-up stem like this https://www.amazon.com/FOMTOR-Bicycle-Handlebar-Suitable-Mountain/dp/B075FL98Y8/ref=sr_1_4?keywords=riser+stem+31.8&qid=1554328430&s=gateway&sr=8-4 but that only raises the handlebars like 3-4 inches. Frankly, at 6'5", to get a very substantial upright position, I would recommend BMX handlebars. I have a 29"er myself, which I put 8.25" BMX handlebars on, and it feels great.

Right now there is a trend of big-retro-BMX-style setups like the SE Big Ripper. A lower-end company called Framed jumped in and made a 29" wheel BMX: https://www.framedbikes.com/products/framed-x-spacecraft-29er-bmx-bike-mens
If you put these SE 7" rise bars on that Kona, it could be cool.
https://www.jrbicycles.com/se-super-big-honkin-handlebar-chrome

Then you'll need a BMX stem because the clamp area on BMX bars is 22.2 instead of 25.4 or 31.8 on MTB.
This stem would be a budget option, but there are others that are in the $45-$80 range:
https://www.amazon.com/Black-Ops-DefendR-BMX-Stem/dp/B004TN08YC/ref=sr_1_5?keywords=bmx+stem&qid=1554328284&s=gateway&sr=8-5

If you don't want a BMX look, that's fine, I get it. Frankenbikes are not for everyone.

Typically frame builders made long head tubes for tall riders' bikes, rather than raising the handlebar height. Check out this old school tall dude's bike. He still has drop handlebars (I guess for aesthetics... but the head tube and seat tube are very tall. https://www.reddit.com/r/bicycling/comments/1bzc88/and_i_thought_my_64_cm_bike_was_large/