https://www.biketart.com/bikes-c1/frames-c33/genesis-tarn-frameset-2018-p43664
It's a bit pricey but it is steel and has a carbon fork which looks quite nice. These people do have a Vagabond but only in M size, which I presume is too big for you. This one is available in S and takes 29er wheels.
My current bike is a Genesis Vagabond. Was after something cheaper as an every day single speed bike.
J
Then the cheapest things that fit and are new are likely to be chinese or taiwanese aluminium, either 6000series or 7000 series. My mtb/tourer/uyility fetch the bread bike is one of those which came from Chainreaction (10 years ago and £80 for the frame without the fork, which cost the same. Chainreaction still has sale sell-offs in that price range but you will have to keep looking to find your size). There was a french bike shop advertising his take on a Vagabond for about half the price (on Le Bon Coin) but due to browser updating I now can't find the link. The frame would probably have suited your spec too, discs, steel, ss capable, ugly as sin and undoubtedly chinese (built by schoolkids probably, exploiting chikd labour
).
When looking for gravel bike frames last winter the thing that turned up cheapest all the time was old models of Vagabond - cheaper than Surly, although not by much.
Other than that it's a question of finding something secondhand. I am keeping an eye out but it is getting a bit late to strap one to Ivo's rack for the return after PBP!