We are thinking about experiencing Grenada’s carnival next year as well as chartering a Grenadines cruise either before or after. Has anyone here done something like this? If so, how was the experience? Any advice ?

We did a week long chartered sail through the Grenadines a few years ago and had a great time. We found it less “crowded” then sailing through the BVI’s. Would do it again in a heartbeat. Carnivals never interested us much so can’t comment about that aspect of you plan.

I haven't been to Spice Mas, but have been to Carnival on Carriacou many times, (going back in March) and if it's any indication it should be a great time. The advice I've seen is to book early as the most popular properties go quick. There should be a trip report from Carnival this year in this forum.
Not much can compare with a charter through the Grenadines, absolutely beautiful.

One more thing, bring old clothes that you don't mind getting covered in used motor oil for the J'ouvert