That's pretty straightforward forward I did a ton of that research for my T50 builds
If you want bomb-proof and fast enough; Then it's the
Marathon Modial DD. It's fast on the road and fast on the gravel and dam near indestructible, goat heads might be a issue but only if you hit one at speed on the side wall. Get the biggest Ethro you can fit on your rimms and run low pressure and they eat up the road. I have mine at 35psi; I just did a 28mph sprint on the T50 this morning and the Silvio had a hard time catching up and passing me on the down hill. Only disadvantage here is they are heavy. The 27.5x2.00 should fit the frame Robert Might know off the top of his head.
Edit: they dropped 650b on the G-One Overland that's a big old bummer it's really the perfect S40 tire in 700c and would have play well on the Q45.
Schwalbe G-One AllAround with the G- Series go as big as you can get in the frame; and run dump low pressure and thin tubes. In most cases you will wear out the tire before you flat the tube on the road. I have the All-around on Devins' trail bike; same pair 8000 miles and 7 years no flats. It's "ok" in the rain but not heavy rain. This tire does only come into sizes up to 27.5x1.5 so it will fit your frame for sure.
Since there's no Overland in the right size, you probably would want the
the Hurricane to fill that void. Again only in the 27.5x2.00 like the Mondial so it could be a tight fit. We need
@Robert Holler to comment on fitting a 27.5x2 on the fork.....
G-One Speed ; if you are not doing crushed limestone it's good on real gravel; limestone is too fine; always consider crushed limestone as "Trail Dry" on Schwalbe charts because limestone after 2-3 years basically is just dry dirt the way it interfaces with the tire; it's just resistant to rutting and washing away which is why they use it. It's NOT gravel. I see this one washing out on Limestone in turns.
If you just want to be annoyingly fast on that thing and you are ok changing a tube every 500-800 miles; get's some Kojaks again biggest dam ones you can fit on should be 26.5x1.35 smaller than the G-Series All-around and higher pressure but about 20% faster at least. Then just laugh as you lay down the road when you can do a 60 degree lean into a turn on what is now a GoKart; at $29 a pair it's kinda hard to bitch about the occasional flat tire; just keep them on the road and out of the rain....... Warning they last about 1500 miles at most.
Here are the charts on the 3 tire series; you can find more tuned tires from Maxus and Kendall but they are hard to find and focus on odd wheel sizes like BMX; for example for my 22" wheels I'm pretty much stuck with Kendal for 700c and 650b Schwable for non racing; and Continental and Victtoria for racing.
Conclusion; Get the G-one All around and give it a go and see how you like them.