For me I would want access to Disney everyday so wouldn't be happy with a 14 day ticket and the 21 day isn't that much more
I agree
I think it also depends if OP stays onsite at Disney's.
Staying onsite will justify going to Disney almost everyday for a couple of hours, to use FP+ and to as much as possible with minimal wait times.
But staying offsite, especially at places like the top of I-drive for example, would make it less efficient (time and money wise) to have a 21 days ticket. A Magic Your Way 10 days would be enough if OP only plans to spend that many days at Disney's.
In my opinion the tickets are the "cheapest" part of the global vacation package. And the more days on the ticket, the less you pay. So I would go all out on tickets. 21 days Disney, 14 days Universal and 14 days Seaworld
When you do the maths, having the flexibility to choose which park on which days is well worth the cost. (of course this is not my money so I'm free to "spend" OP's dough ...)
The main advantage of having a 21 days ticket is being able to pop in for a quick visit in a disney park at the end of another park's visit
also, planning rest days is a good idea, it's even necessary when you go for 3 weeks. But I've found that half days at the parks were more efficient.
Mornings in the parks when it's quiet and the heat is bearable, and afternoons at the hotel or doing other activities, and maybe a quick visit to a park in the evening for fireworks.
When you have a 8 days ticket you won't do half park days, you will probably want to spend the whole day there, because you only have 8 days. With a 14 or 21 days, you're free to call it a day come 2pm and do something else. It's not a waste of money. And if you plan rest days, it will probably be more tiring to do WDW in 8 days with a rest day in between, than doing 16 half days at disney and rest the other half of the day.
You'll do the same (maybe even more because you'd have 2 times more FP+ and 2 times more quiet times in the morning) and it will not cost that much more.