Welcome to CF Anjulis.
We did a 2 week charter in 2013 from Rodney Bay, St. Lucia to Grenada and back covering the area you mention. I think I'd save the $1000 and return the boat to Marigot Bay. We did Soufriere to Chatham Bay on Union Island in one long day, so you could do a one day run from Marigot to Canouan or Mayreau then work your way back a bit slower from there.
We only did a day stop at Tobago
Cays then went to Saline Bay on Mayreau to anchor
for the night. But a night at Tobago Cays would be very nice! There are some shore suppers offered by the boat boys that would be worth trying out.
On the way back I recommend you spend at least a day at Bequia. It's a nice anchorage and good restaurants & shopping
We anchored off Wallilabou Bay on St. Vincent on the way back. There are some security
concerns mentioned on St. Vincent but we were a larger group so not concerned.
The run from St. Vincent back to St. Lucia was a bit brisk off especially off the north end of St. Vincent, but depending on your boat, should not be that big a deal.
Depending on your timing, you could do a stop at Soufriere & the Pitons on the way back also. Get a boat boy to help you grab a mooring
at the Bat Caves if you can. Or a mooring
right at the Pitons. We've also tied up stern too right on the beach in Soufriere.
I highly recommend you get this book - Sailor's Guide to the Windward Islands
by Chris Doyle - to read up ahead of time. It's the gold standard for reference.
The Sailors Guide to the Windward Islands: Chris Doyle, Virginia Barlow: 9780944428993: Amazon.com: Books
Anyway, those are my thoughts. Good Luck!