First Gravel Bike. What to Expect and Future Upgrades.

Hi everyone. I want to buy this bike, would be my first gravel bike. Currently I own an aero road bike which is my main training rig, and a XC MTB which I also use for racing, but I use the road bike far more often. Since I have both bikes pretty high end specced and don't want to "waste" good components and tires, I was looking for a new addition to the collection, and this bike was offered to me for roughly $850, in my market this is an insanely low price because gravel bikes are extremely rare, so I'm stoked to pull the trigger but I know there are several hiccups with it.

First, it's not really a gravel frame, I looked it up on the official Mosso page and it's a road bike frame, but somehow they fitted 38c tires on it. Second, I'm not sure exactly what size it is, obviously I'd go to see the bike and try it first but from what the shop told me it has a 52 cm top tube, this coincides with Mosso's official geometry spec, and after doing some comparison, the bikes are very similar so hopefully not big issues there.

I want this bike to take me to more places than my road bike can take me. I obviously could go with the MTB but I find it boring and slow to do long days in it. I know the 38mm is a bit of a limiter too because gravel bikes now accomodate well over 45mm and some even 2.2 mtb tires, but is it really that limiting for mainly fun riding and general endurance training?

Anyways, what else should I upgrade in the future? I hope to install tubeless tires since tubes suck, but that's kinda it.