Congrats on our first cruise! You will be hooked
I would definitely do the concierge lunch as you will 1. get better quality food than Cabanas, 2. Be able to meet the concierge host and get a wealth of information, 3. Be a little pampered and welcomed aboard the cruise! (i don't know how long you are traveling to reach port, but we LOVED being able to sit down and have someone serve us lunch instead of fighting the crowds of Cabanas after extensively traveling to get to the ship). Cabanas is not going anywhere and you can/ and likely will access it for the remainder of the cruise. You could be in and out of Tianas as fast as you choose... just let the servers know, but having just done it, go for it! IT went much faster than normally eating in the main dining room- likely because there were not that many people there. It was such a pleasant surprise and a great way to start the cruise. You could be in and out in less than an hour and the value of the atmosphere and education from the concierge hosts, I think, would make it worthwhile.
And back to number 1, the food will taste better in Tiana's, and even more so in the lounge. We would rather have smaller plates of more flavorful food, and Tianas (but especially the lounge!) provided that for us. My 9 year old refused to eat in Cabanas early on in the cruise, as she said "Mom, it's pretty, but has no flavor, can we just go eat in the lounge". (I completely realize this is a statement of how we eat at home... so this is not a judgement against anyone who loves Cabanas... for us, it reflects the style of how we eat on a regular basis and the small bites in the concierge lounge were more appealing to all of us than large portions in Cabanas. She would rather have a small dessert from the lounge packed with flavor than any of the desserts in the main dining room... )
Enjoy your cruise! There is so much to do, you can run around having FOMO, (fear of missing out), but for that lunch, it is for you! And sitting and savoring just being there on the boat is such a wonderful way to start the cruise!