Help--first ever cruise

My wife and I are taking our first vacation in the 13 years we have been together (long story). We have never done a cruise, but have decided on VV due to the adults-only, all-inclusive aspect.

Anyway, we want to book in the next few days, and have narrowed it down to the Key West short cruise (maybe just to get our "sea legs") and the Cozumel one. Looking at late March/early April this year, if there are any deals. We just discovered this group (Reddit has everything!) so I wanted to throw out a few questions (I know each of these could be its own post):

--Everyone on here seems to be using TAs. Why? Am I missing anything in particular if I just book?

--I saw a post where someone was doing a Sea Terrace room, about ~3k, and everyone responded that that was really high. But those are the prices I'm seeing. Am I missing something?

--We are deciding between a Sea Terrace and a Rockstar room. The cost almost doubles, and it seems just to get a bigger balcony. Is it worth it in your experience? Is there a huge benefit to the personal concierge/agent, or something else I should consider?

--I see some slang I can decipher in context (TA, sailor Loot) but others I cannot. What is MNVV? OBC? Like I said, just got to this group and have been reading like crazy but missing some details.

--Looks like VV is non-refundable. Does that change through a TA? Does anyone get travel insurance?

--Any general tips/advice, or questions I should be asking?

TAs feel free to message me, and thanks all in advance.