The point still stands that the message for a "slimmed-down monarchy" was geared to say that any grandchildren of the monarch will not become working royals, regardless of their titles.
I'm very much looking forward to HM's first Trooping of the Colour as King. I wonder if the King will continue HLM's tradition of being more generous with the balcony appearances for the birthday celebrations.
If he is as generous (or even slightly more generous) he could use the same cut-off point: cousins and their descendants (although in practice he might want to discourage some in participating - previously not everyone who was invited participated yearly either).
If so, the following people would be eligible to appear on the balcony:
Charles & Camilla (2)
William, Catherine, George, Charlotte, Louis (5)
Harry, Meghan, Archie, Lilibet (4)
Anne & Timothy (2)
Peter, Savannah, Isla (3)
Zara, Mike, Mia, Leah, Lucas (5)
Andrew (1)
Beatrice, Edo, Wolfie, Sienna (4)
Eugenie, Jack, August (3)
Edward, Sophie, Louise, James (4)
David, Charles, Margarita (3)
Sarah, Daniel, Samuel, Arthur (4)
--> 40 people (at the moment: pretty soon adding the newest Brooksbank)
In addition, if he follows his mother's example per above, I assume he will invite the other royal highnesses from the previous generation (of which at least the Duke and Duchess of Gloucester are likely to appear) - but I see little reason to continue to invite all their children (6th degree blood relative/second cousin) and grandchildren (7th degree blood relative/second cousin once removed).