OK, that’s not too bad :-)
Do you know the various ways of editing keys?
If you drag a key, you only copy the current level, but if you right-click and choose Copy Key, you can copy and paste all levels in one operation.
Or use the Inspector to go through all the different levels for one key.
Or activate Keyboard > Quick Entry Mode to go through all the keys (described in the PDF manual point 6.6.3 which is on page 30, that’s the page marked as page 26.)
By the way, I encourage you to put your custom layout in a collection/bundle.
It allows you to give it its own icon and assign it to a specific language.
The first is nice and makes it easier to spot it on the menu.
The second seems to have important effects on the behavior of some spelling checkers – although I must say I don’t have any experience with that and don’t know the details.