Cheaper and available is critical of course. $80eur is the difference in the XDR driver in the GR model. If you’re not running road tires as a 2nd job, the aero point is moot. If you already own an ER wheelset, getting the DT wheelset is a slam dunk. That’ll let you move cassettes around as your needs change and save money on cassettes.
Not sure what you’re full garage looks like, but if you just have the Endurance and Gravel bike, I’d put the ER on Gravel duty and get something more road-aero orientated - maybe the ER1600 32 at the $400eur or a LIghtbicycle wheelset at the $800 price point.
On the C vs G series 2mm topic - the 2mm interior /exterior width is totally meaningless here. Once the tire is wider than the rim, aero is out, so all that’s left is how round or oval the tread profile winds up and how the sidewall folds. 2mm difference on a 40mm balloon you’re mounting on metal clamp is nothing - that’s about a .5 - .6mm difference in resulting tire diameter (1.2%). They’re in the same neighborhood on a Gravel/MTB setup where you’ve got a tire 10mm+ wider tire than the rim (you’re talking about a sidewall cantilevered either 5mm or 6mm over the rim edge - either way it is cantilevered over the rim edge a moderate amount).
You can run a full MTB tire on either the C or G series width, it won’t make a performance difference in any capacity (cornering, ride, tire pressures, etc). The width here is only going to impact the tires shape when mounted.
Most MTB rims are narrower than these, so the reason for having the wider rim is to be aero-ish with the right tire (25c c-series vs 28c g-series) , marketing, and some crosswind performance. Vs a MTB rim, either of these will be heavier, need less spokes, hold more PSI, and handle crosswinds better.
BRR tested a few widths on their rig, and they saw some tiny differences under these conditions ( https://www.bicyclerollingresistance.com/specials/rim-width-test ). Pinkbike tried to test larger increments on MTB terrain and didn’t get a good repeatable test result - (7sec on a 180sec course with a ~6 second margin of error after he practiced) Comparison Test: Are Wider Rims Better? We Try 4 Different Widths - Pinkbike - the final tire shape on the rim was more important than the width. Anecdotally, I’ve gone from 24mm wide ext to to 32mm wide on the same tire and couldn’t tell you the difference.
I’d still not waste money on the noiser 36t ratchet, at 27kph at 90rpm on the pedals, you’re looking at a 4deg (25mm of free travel vs 12mm) difference between the 18th ratchet and 36t (that’s probably really a 2deg difference avg). 36t doesn’t get you down to zero and 18t isn’t anywhere near bad for most gravel riding. Not worth the noise tradeoff.