I have created a "Photoshop-style" channel-based luminosity masking macro following the instructions provided in previous comments. Credit goes to Depa and smadell.
Macro is created and tested in Affinity Photo 1.7.3 running macOS Mojave
This is not to support any debate which luma masking is better or worst just for technical curiosity. I have discovered a shortcomming in Affinity Photo 1.7.3 as the channels can not be named for the macro. If the macro is not recording they can be named properly. So mind the order of the channels such as: Highlights 1-6, Shadows 1-6, Midtones 1-6 from top to bottom.
Use of the macro:
Open your image.
Import the macro. View / Studio / Marco / Import
Run the macro and / or save it wherever you wish
Open the channels panel (View / Studio / Channels)
There you have all the 18 masks in the lower part of the channels panel
Pick the preferred luma mask from the channels panel and right click 'load to pixel selection'
Layer / New Maks Layer
There you have the chosen luma mask loaded to your masked layer
Proceed with your edits as you wish
Do you really need 3x6 channels? Probably not, but if in any case you find use for it, there it is.
Usually 2-3 luma masks are used per image depending on the luminosity info of the photo and the complexity of the edit.
Jimmy McIntre is a master of all luma magic worth checking out some of his tutorials on masking, blending, orton effect etc
6-level H-S-M Luminosity Masking : Spare Channels.afmacro