For about $300 plus wheel build you have 2 options.
If not through axle but with disc: Sturmey Archer CS-RK3
If not through axle but rim brake: Sturmey Archer CS-RF3
You can operate either hub on road bars via a Paul 31.8 adaptor & SRAM triple mountain shifter. I have one bike set up this way. 20-140 gear inches using the 42 ring & 11 speed, 11-42 cassette you have already.
If not thru-axle & you'd like to spend about $450 plus wheel build instead, you can look on the used market for a second hand SRAM dual-drive. Repair parts are seldom, if ever available & my understanding is the 3 speed hub shifter is flat (straight mountain) handle bar only. But they work good.
If your bike is metal with a BSA bottom bracket & you've got about $900 floating around: Schlumpf mountain, speed-drive, or high-speed drive are options. It is not suitable for exceedingly filthy enviromnents. A speed-drive with a 34 tooth ring would give you about 0.75:1 & a top gear of 56-11 & still look like a 1x. You can get chainrings as small as 27 teeth with the "dash '08" varient. My Dahon has a speed-drive with a 34 ring mated to a Nexus 7 with a 17 cog. 24 to 97 gear inches.
If you are made of money & the above options don't tickle your fancy: Rohloff in any way you see fit. My touring bike has one. 17.5 to 93 gear inches. You can pair a Rohloff with a double crankset.
Or you could just go old school with an older model triple, 8 speed Claris, or 9 speed Sora or 10 speed 105 shifters with a 24(or28),38,48 triple & a 11-34 cassette. Road shifters, matching road triple front derailleur, & a long cage non-Dynasys mountain derailleur. That's how it was done back in the day.
Lots of ways to skin a cat.
BTW: 1:1 ~29-31 gear inches, is what road riders use locally for roads. For gravel here in the PNW, about 0.6-0.75:1 or ~16-20 gear inches is common. People who are not me seem to happily top out at about 18-25mph & be ok with it that way.
base2
(Who continues to be unimpressed by GRX, Di2, thru-axles, press-fit bottom brackets, or 1x.)
Last edited by base2; 07-05-22 at 03:33 AM.