Originally Posted by badger1

I'm in Canada, so yes: it appears that this new Trek FX is intended to compete directly with the Sirrus X 5.0.

I own/ride a Sirrus X Comp carbon ('19), which is essentially the same as the 5.0. It's a great bike, as I'm absolutely sure would be the FX.

On your questions.
1. FS vs. rear Isospeed: personal preference, I would think. I looked long and hard at the current carbon FXs, but I went for the FS precisely because I'm especially sensitive for various reasons to 'shocks' up the front end of the bike. The FS is very effective. I know that the rear Isospeed is as well, so you just have to decide which matters more to you.
2. Tire clearance: the Sirrus X frame will clear up to 42s. I run 32s on mine (road riding). I'm not sure what the clearance is on the FX; you'd have to check that with a dealer, if it matters to you.
3. Gearing range and quality: SLX (mtb) and GRX 11spd are pretty much equivalent in quality. For what it's worth, the gearing on the Sirrus 5.0 (42x11) is actually a bit 'taller' than that of the FX (40x11). Neither will affect your 'top speed' in any practical sense.
4. Geometry is different, as you say. Again fwiw, I find the positioning on my Sirrus to be a perfect compromise between 'aggressive' and 'upright'. Personal preference, obviously, but to my mind this is the crucial difference. If your preference is for a slightly more 'upright' position, go with the Sirrus. Yes, you can futz around with stem angles and bars to make the FX more 'upright' if you want, but if that's where you're going to end up why not just get the frame that 'fits' your preferences more effectively in the first place.

tl;dr summary: these two bikes are much of a muchness, imo. I don't think you could go wrong with either one. But if it's possible in the current climate, I'd wait until I could test ride; one will likely 'feel' more like 'your' bike than the other.