This little female is really puzzling me and she finally let me get some pictures last weekend.

• Host plant: Shasta daisy (Leucanthemum × superbum). Only observed on this species numerous times over the past week or so (we have multiple clumps of Shasta daisies). I tried to catch her, but she wasn't amenable.
• Size: ≤5 mm. I know for certain that she was less that 5 mm in length, because I measured the plants' central discs and they topped out at 18 mm in diameter.
• Date & time: July 21, 2018, 3:00 pm

Is she maybe a Stelis? She didn't fit any of the keys I tried for Dianthidium or Paranthidium—and it looks like she doesn't have much of a scopa. We do have an enormous mixed population of Heriades, Megachile, and other Megachilidae nesting in our yard this year (I've been meaning to post pictures for identification), and our cuckoo bee population in general has just exploded (if she is a Stelis, she'd be the 6th cuckoo species this year and 8th overall). I also have a ton of extra photos not included here, so if you need more, please let me know! Thank you!! ETA: Apologies in advance if this is too many photos, I just wanted to make sure as many features as possible were visible.