The best way of slimming down the official family is probably to simply limit future titles to the children of the monarch and the children of the monarch's eldest child.
For example if Harry has children who are prince/sses then the family will be larger than it would if they were without these titles.
Except why does the monarchy need to be slimmed down?
Last year, the Queen, DoE, Anne, Wessexes, Yorks, Gloucesters, and Kents performed 2,463 duties according to Bertie's count. If the monarchy is to be "slimmed down" during Charles' reign then these people will not be doing any duties at all, either because they've passed on or been retired.
That means that Charles, Camilla, William, Catherine, Harry, and any hypothetical wife of Harry's will either have to cover those engagements or cut them out. Just to do that, each of them would have to perform 410 duties a year - which is fine, until you consider the fact that that would be on top of what they're already doing.
That's also not taking into consideration the fact that some of the numbers were down last year owing to health problems.
If you divide the total number of engagements done last year by these 6 royals (one of whom is a hypothetical), it means that they would each have to perform 614.5 engagements annually in order to maintain what is being done now. Without Harry's hypothetical wife, it's 737 engagements each.
Or instead of slimming down the monarchy the status quo could be maintained, with the monarch, consort, and the monarch's children performing the bulk of the engagements while others further down the line perform a smaller number of engagements.