I have a 2020, I’m generally happy with it. The price you have found is a good deal. I noticed it doesn’t have the sunroof, parking sensors, or LED headlights. The LED lights modernize the looks of the car IMO and also provide much better lighting. They cannot be easily retrofitted, but you can swap the OE bulbs for LEDs.
I noticed it does not have the AMG-line package, which is the only way to get genuine leather seats IIRC. If I’m correct, then this one has MB-Tex/ARTICO aka leatherette. That’s not a bad thing, I like the durability of the ARTICO and it’s easy to clean. It also doesn’t show wear nearly as quickly as the Napa leather in our GLE53, which creases on the bolsters very quickly.
I would go check it out in person though just in case there’s something wrong and that’s why it’s priced relatively lower. Check CarFax as well and visually inspect for signs of damage/repair such as differences in the paint shade, panel misalignment, or significant differences in the reflection off the paint. Cheap or inexperienced body shops can struggle to blend new paint with existing or may use cheaper product to cut costs. In any case, make sure you can get as much info as possible to know you’re not getting scammed.
If the seller is representing this as “fully loaded”, I would be skeptical as to what else might be exaggerated. The aforementioned features are all optional, as is metallic paint, 360° camera, and others. The alloy wheels on this particular one are also the included ones, i.e. the standard ones, not optioned or upgraded further. The door handles also don’t have a chrome strip on them from what I can see, which means no Keyless entry or push-button engine start. Need to use the key fob for both.
The VIN decoder showing this as a 2021 is also weird unless you’re using an inaccurate one, try another one and see if it still says 2021. This car is most definitely not 2021, which would indicate VIN fraud or swapping to me i.e. seller wants to hide something about its history (stolen, salvage title, rebuilt, etc.)
If everything checks out when you see it in person, then I’d take it because that’s a good price. Those missing options aren’t a big deal unless those are features you want, in which case look for something else as MB is very difficult and expensive to retrofit on.
Good luck!