You need to be looking at stack height.
Its not just head tube length, its also fork length and internal or external headset, plus how much the bottom bracket drop is below the wheel axis that defines your height relative to the bars.
You need to be at around 650mm, otherwise your reliant on headset spacers, flipped up high rise stems and bars that go up from the stem which have just come out.
In my wanderings (I’ve a bulging disc) getting a high stack height is the difference between having a back ache the next day and not on a lower front end.
As a pure road bike, I found the Specialized Roubaix carbon had about the highest stack I could find. The big carbon tubes and head area disguise it, but on a 58/ 5’11” bars are level with the stem upward.
For the gravel bike, a Salsa Fargo gets you high. Its 29er derived, so its got a 480 carbon fork (unlike a road 390 fork), so you’re 4 inches higher before even looking at the head tube. Stack height is 671mm on the large with 38 reach. You can put more comfortable tyres and a 100mm fork on it to take stress off your shoulders and back. If you’ve got the cash the titanium Fargo has the most comfortable frame. The CutThroat is the carbon version, but supposed to be stiffer and its slightly shorter in the head tube, so would be less good for you.
Add a Thudbuster, a Brooks Imperial, some thick squidgy bar tape, a SID 100 and a decent spread of gears and it’s like riding along on a Chesterfield. Any tyres fit the new framesets. Any. Older non boost bikes take a 29 x 2.4 rear and upto 29 x 3 front on the Firestarter fork, if you want to go bigger than a typical gravel tyre – not just knobblies, Schwalbe do some larger light road tyres like 2.25 G-Ones for max cush.
If you still get trouble, don’t discount an offroad trike just because they’re a bit out there. They’re alot of fun (remember go-carts). You’re completely lying down, and when you stop to admire the view, its like you’ve brought along your own deck chair. It’s surprisingly civilised. I had a go on Dad’s Ice Trike and liked it.