Keep in mind as things stand now you will have to be tested for covid to enter SVG and again to enter Grenada. We use a nurse that comes to the marina, your hotel or villa in St. Lucia with results ready in the evening if you test in the morning, cost $130 US. On Union island there is another nurse that will do the PCR testing, results available in evening for $60 US.
The testing timeline may impact your schedule. I'll try to get the contact info for the nurses and post them here but it might be a few days and the requirements are always subject to change.

Drink all day at home, your friends worry about you; do it on vacation and they say "what a good time you're having". Save your friends needless worry, travel more!