So, I'm going to partially answer my own question, as I found an earlier topic from 2017 with basically the same question:
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The Affinity Photo Pixel Tool is (more or less) the equivalent of the PS Pencil mode. When selected, the AP Pixel Tool has an "Alternate" option on the Context toolbar. Set that to "Erase" (or "Background Color"). This changes the effect of the tool when the 'alternate' key is held down.
A bit confusingly, on a Mac the 'alternate' key is not alt/option but CMD. However, the Status bar at the bottom of the window should tell users which key invokes the 'alternate' action, so it is not too bad if you refer to that.
Anyway, with appropriate Opacity, Flow, Blend mode, Alternate mode, etc. set in the Context toolbar, the Pixel Tool with the appropriate keyboard key held down should produce hard-edged, aliased erasures.
Source topic
So with some digging around I found the answer, though as a piece of feedback for the devs: I did not find it entirely intuitive to find this under the pixel tool. I might be because I'm just getting accustomed to a new workflow, but I don't think it would be entirely intuitive for complete newcomers either.
I think adding a reference in the erasing section of the help would at lease give a hint that the pixel tool also has erasing features?
In the image below the desired effect (you activate alt mode by holding ctrl)