Fentuz

Have a look at Kona Sutra and Cinelli Hotbootleg GEO. They are set for packing packing
they run 11-42 x 36 or deore twin chainring.

I run mine on 11-42x36(eagle range) and get 24gi in 45-622 and 23gi in 50-584. when a current cassette is worn out, I'll go for Shimano SLX 11-46 to get 22gi in 45-622 and go through the wintery muddy rock hills

If you want something below 20, you will need a mullet setup (gravel bike fitted with XC mtb transmission) which is either expensive (see DI2 or etap) or a non standard product.
you would need to run a 11-50 or 11-52 cassette with a 36 or 34 chainring (or lower if you don't need flat speed). there are hacks for Sram where you can mix derailleur parts for eagle GX range (and above) and rival range (and above):
one of them is a cage swap: you have to fit the eagle cage on the rival derailleur.
another one is a cable guide swap: you have to fit the rival cable guide (dictating cable pull ratio) on a eagle derailleur.
These use genuine manufacturer parts but are not manufacturer approved..

Or, you fit a goat link between the frame hanger and derailleur. clever but I am personally worried this could increase the risk of bending the hanger when there is a unwanted collision.