When the Create button is visible over a subgrid, it's nice to use it and users will see it a lot easier than having to go in the three dots to see it.

My proposal is to have an option on subgrids with just a checkbox, once checked, the button for Create a new record of this subgrid will be always visible. Enventually, if responsive doesn't let enough space, we can just put a square button with a + inside without the full text.

Also, specifically for N:N relationships, the option should be also visible, it just applies to "Add existing" instead of Create.