I went through the same thing. We didn't put a deposit down for anyone because I didn't want to risk it. I made my own invites so I had everything ready and once I got pricing, FI spent a couple of days making them and sent them in the mail. Our guests received the invites with all the info and still had 3 weeks to decided if they wanted to put a deposit down. We weren't expecting a lot of people, but the last day to put the deposit down is actually today and so far 40 have booked! We have quite a few people that are going to hold off and book later this year. The people that wait just might not be able to get the group rate later on, but when it's closer to the date they might also be able to get a last minute deal.
I don't think you really have to worry about which tour operator to use, they're all pretty much the same. I'm going with Sunwing/Signature but have also travelled with Signature and Transat. They all provide pretty much the same services, might just have a few different perks.