You are hitting on probably my biggest problem with the camera. I'm not even complaining about Pentax specifically here; every system which has APS-C cameras that share a lens mount with full frame cameras frustrate me, because APS-C is a really awkward crop for full frame lenses. If you like telephoto, especially short telephoto, great. And I actually really like the 70-75mm range. But it's simply not versatile enough to stick a full frame 50mm on APS-C and call it a day. I did that for years with my Nikon D40 when I could only afford one Nikkor 50mm prime. It's a sometimes awesome combination but also often frustrating and impossible for close quarters with people.
Now, Pentax does have some really nice APS-C lenses, especially the DA Limited line. Most of them aren't as large maximum aperture as you mentioned, so if you really must have the shallow DoF all the time, then they're not great replacements. But if you stop down at all, then they're equivalent lenses, plus you can shoot crazy high ISO on the Monochrome no problem. The 15mm and 21mm DA Limited lenses, and then the cheap 35mm 2.4 would be a nice kit.
But back to my problem: I have a K-1 II, and I love it. But I'm forced to think of these two Pentaxes as two different formats. The only lens I own which is actually versatile on both is the FA 31mm Limited, which is a great lens, but it's expensive and according to my calculations, it would have to be sold in the short term to keep the Monochrome, which defeats the purpose. The other lenses I enjoy on the K-1, the 35-105mm 3.5, 35-70 3.5-4.5, couple of 50 1.7 versions and 50 1.4 are not useful everyday lenses for the APS-C. That's really frustrating. I owned the FA 35mm f2 for a while, but it's not really sharp enough optics to function as a good nifty fifty equivalent on the APS-C.
If I sacrificed other gear to keep the Monochrome I wouldn't be able to afford the 15mm and 21mm DA lenses just for the Monochrome. I could get the 35mm 2.4 but then I'm putting a very budget lens on a high-end body for a while.
Oh yeah, if you want to consider the Monochrome with a standard equivalent fast lens, the FA 31mm Limited is your best bet for really nice optics, the older SMC version isn't horrible.