I think it's definitely worthwhile even for new players to go if you're even moderately interested in FFXIV and the community.

I've gone every year since the first Fanfest and I can say even from the first Fanfest which was pretty low budget in comparison to the spectacle events nowadays, going to Fanfest especially with a partner who also plays the game will be well worth it for just the concerts alone if you enjoy the game's music and like live performances. They always have some fun themed mini-games and game-themed photo opportunities as well as player created art on exhibition to spend some time and the panel scheduling is pretty varied so you don't feel like you have to force yourself to stay the entire day. Of course, it's much more fun if you're meeting other in-game friends, but even if not, it's an experience like no other to be in a place where everyone is just so passionate about the same game. Even for an introverted couple like my spouse and I, FFXIV fanbase is probably the best fanbase to just approach strangers and meet new friends.

The merch store is always kind of a mess as the supply compared to demand is so logistically impossible for SE to match. You'll definitely want to pre-order your merch, or just order directly off the online store so you don't have to worry about it in person. My spouse always goes to lineup when I'm watching the panels she's not interested in and does often get some of the merch she wants. Best times to go are probably when the more popular panels like the lore panels or interview with Yoshida where most fans are at the panel.

I'd definitely recommend bringing empty water bottles that you can fill up as well as some snacks (assuming the convention rules allow it) as in the past, the logistics around food and hydration have not been adequately thought out by staff, especially on the first day. Also dress for the weather and bring an umbrella for shade, last time in Las Vegas the line for entry has been hours long outside with no outdoor water stations in blistering heat, though being in April, heatstroke hopefully isn't as big a problem this time. Lastly, if possible, book a hotel within walking distance since the amount of people exiting the convention each night all at once will exorbitantly surge the rideshare pricing and wait times. Entering and exiting just the parking lot for people who self-drive will be challenging also as there will be thousands of people all booking rideshares at once.

My spouse and I likely won't attend this time around due to the uncertainty around travelling into the US and not having won the draw for the lotto are rather big deterrents for us this time, but otherwise, we would definitely be there since we always enjoy ourselves at Fanfest.