No actual hands on experience but lots of reviews/research on the GRX series. I'm actually really looking at getting GRX for a new to me cross/gravel bike-I would say for gravel/rough surface the big advantage is the clutch in the rear--that runs through the whole GRX series so it would seem that the difference between 400 to 600 is 10 speed vs 11 speed. You're currently on 10 speed so unless you are looking to switch to 11 speed the 400 seems like a good fit.

My new gravel bike has a GRX 400 and I love it. I know it is a step down, but it seems similar in quality to the 105 set up I have. Where it really shines is in the shifter set up. Nice to hold on to, easy to shift and the breaking seems much easier. I have never had to use the clutch because even with it not engaged it seems stiff enough that chain slap is not a problem for me.

I ride 105, Ultegra and Dura Ace on my Road and Tri bikes and as I said, this seems to be the same quality. I briefly had a gravel bike with Claris on it and I hated it. Chain slap all the time and problems shifting under load. The groupset was probably the reason I got rid of that bike. My new bike with the GRX 400 is a pleasure to ride. I got it with the idea that I could upgrade it later when I could, but frankly I'm pretty happy with it as is.

Test rode it at lunch, I really like the shifters. Seem beefier and more ergonomic. Shifting seemed smooth but nothing better than my current Tiagra.

Diverge seemed a lot more comfortable and nicer than my current setup especially for the price ($1,900).