Kitting looks really good overall. I have a few suggestions/recommendations, though:
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The Base kit supports a variety of boards, from 60 to TKL.
The R1 1u Backspace key in the base kit should be changed to Delete.
R1 Delete is required for various 60%, 65%, 75%, 1800 etc. layouts, whereas R1 Backspace isn't really required for anything and can be substituted with Delete.
I think including 2× 1.5u Super and a second 1u Alt for Mac / OG HHKB bottom row users would be more useful than the 1.5u Shift.
The 1.5u Shift can be moved to the 40s kit since some 40s boards use that key, and they're more common than the non-40s boards that use it.
That being said, since you have 1.5u Super keys in the icon kit, you could argue that including them in base isn't necessary. That may well be true. However, I would probably move the 1.5u Shift to the 40s kit regardless.
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The 40s kit supports many Ortho and 40% boards.
2.25c and 2.75c spacekeys (as well as a second copy of 2c) should be added to this kit, as these sizes are crucial for 40s users. You don't want to be forcing them into also needing to buy the spacebars kit.
This kit could do with a second 1u Ctrl key (as well as a second 1u Alt if you opt not to include it in the base kit).
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The International kit covers most European layouts.
The R4
;: key in this kit is in there by mistake. It's not used in any international layouts and should be removed.
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The Icon kit allows you to change up your modifiers' look.
This kit is missing the second 1u Alt icon key to go with the two 1.5u Super icon keys.