HTupolev

"Total capacity" in Shimano spec lingo just refers to chain wrap. The caveat to exceeding it depends on where you err on chain length: err long and the drivetrain will go slack in the smaller cogs when you're on the small ring, err short and the drivetrain will pull taught when you're in the big ring and bigger cogs. The latter can catastrophically damage some drivetrains, so a lot of people choose to err long.

I say "alright" because it's true that it's not perfect. I'm not familiar with its specific ins and outs because I haven't ridden it, but he does say that his road drivetrains shift better. It doesn't look like he's having any notable trouble with it when riding with him.

I haven't checked the wrap of a GRX derailleur carefully, but I'd be surprised if the wrap specs aren't somewhat conservative.

GRX rear derailleurs (and all multi-chainring Shadow rear derailleurs) use an unsprung b-pivot and a jockey wheel concentric with their cage pivot. Vintage SunTour long-cage derailleurs also had those properties, so here's a comparison with one of them:
A circa-1985 SunTour XC 3600 derailleur has a 10-tooth tension pulley, and is about 3 1/8" from the pivot to the tension pulley. An RD-RX810 uses a larger 13-tooth tension pulley, and is an absolutely enormous 4 1/4" from the cage pivot to the tension pulley. The XC claims a total capacity of 38 teeth; the RD-RX810 claims a total capacity of 40 teeth. This is a ridiculously small discrepancy considering how much larger the latter derailleur's tensioning system is.
(And while it's arguably a muddled comparison because of the differing derailleur topologies, here's a comparison with another modern Shimano derailleur: the Alivio RD-T4000 has a 10-tooth tension pulley and roughly the same cage length as the RX810, but its specified capacity is 45 teeth.)

There's definitely some slop when it comes to cog clearance. The 34/36-tooth claims are based on what's sold alongside the GRX derailleurs in official-spec packages, and perhaps also correlates to what their geometry is optimized for, but b-pivot adjustment produces a pretty large range of jockey wheel positioning.

The 48-32 with 11-42 cassette isn't anywhere close to the most extreme out-of-spec work I've seen done with a GRX drivetrain. I was using it as a relevant example as what I know for sure is feasible within the drivetrain that the OP is using.
The craziest 2x GRX setup that I know of is on another friends' bike: he's running a 48-32 crank and an 11-50 cassette with an RD-RX810, a hanger extender, and a very long chain. I can't speak to how great it works; I haven't witnessed any trouble with it, but I've only graveled with him a couple of times, and only on mountainous terrain where the potential problem cases wouldn't necessarily have been getting put through their paces.

I wasn't saying that you'd need to. My point was that 1x wouldn't necessarily solve anything because most 1x gravel drivetrains aren't really spec'd to tolerate lower gearing than the OP's setup.

I usually don't, but I tend to make exceptions when there aren't many "correctly"-spec'd options.

Although in this case, there is at least one rear derailleur on the market that is theoretically correctly-spec'd for the use case: the S-Ride RD-M520C. It's nominally "11-speed road", but Shimano's 10-speed road drivetrains since 2015 use the same actuation ratio.