Consider removing the mod-colored R1 1u
\| from the base kit. This key is not commonly used. The prevalent split backspace configuration is:
The minority of users who like putting tilde first still have the option of doing so, just with alpha-colored keys.
I think you have too many ISO Enters in the base kit. Consider offloading some of them to the additional kit or dropping them altogether. On a related note, I don't think ISO-UK support in base is a must as those keys are only used by about 3% of all users.
As for the additional kit, you can drop the 1.75u space. Not only is that key very uncommon even among 40% boards (it isn't used on anything >40%), but there is only a single board (Vortex Core) that uses one 1.75u; the remaining 2–3 boards (JD40/45, Prime_L) all need two. Plus, the kit doesn't have the keys required to cover those boards anyway.
Speaking of 40% support, from what I've seen, the consensus among 40s and ortho users seems to be that offering just 2–3 keys is not useful at all. “Designers should either offer proper support or not bother at all.” Price isn't as much of an issue; proper compatibility is. As it stands, I don't think any 40s or ortho boards could be correctly covered with the minimal support in this kit (other than Romeo and Equinox, which are designed to be base-kit–compatible anyway).