Shimano GRX groupset with mechanical brakes for CX/Gravel
The 2019 European Champion thinks Ultegra is quite fine for off-road https://www.cxmagazine.com/bike-profile-yara-kastelijn-stevens-super-prestige-cyclocross even for the 2019/2020 season. And yes, she is using 1x, although with hydraulic brakes and Di2 (electronic shifting). Many professionals are using Dura-Ace with 1x.

There is a difference in the brake lever (perhaps only the Di2 lever, see the comments) in the GRX shifters to allow more efficient braking from the hoods, but it is not considered essential yet. Many people do without. All other top non-GRX shifters should be pretty similar as the Ultegra.

I do not know if the the unconnected left shifter would feel weird, as Michael suggests, but GRX would be the same. You have the option to connect the GRX shifter to your dropper post. But if you don't have one...

As for the Apex, the front part of the groupset is independent, so you can have it and as long as you are only using it for braking the only wierd thing that remains is having two shifters with a different shape o one bike. It might feel or look strange. If you change everything to SRAM, you would have to be switching shifting between bikes, but many people manage with that.