We’ve booked accommodation, flights, car hire and park tickets separately. Made sure travel insurance was in place first and it covered separately booked items as a ‘trip’.
We used Virgin Holidays just for the car hire. They were competitive on price, you pay in the UK and depending on which airport you arrive, the car hire firm had a dedicated desk for Virgin customers. Never got the upsell at the airport when booking with Virgin.
We booked flights with Thomas Cook - sadly no longer possible - as they were the cheapest and had a new fleet of planes.
Booked condos or resorts directly with them over internet.
Park tickets from uk site attraction tickets direct ( they’ve changed name slightly these days I think )
Going for three weeks , trip of a lifetime you say... look to have a day at NASA Cape Canaveral. Book the Lunch with an astronaut lunch.
Make a round trip? Fly back to NY or Washington DC in the last week and spend a few days there?
Remember you’ll all need ETSA , careful you use the official US Govt site and not a scammer site.
Second thedibb.co.uk website. There used to be someone selling their own laminated fold out map of the park area, get one if that’s still going