You're probably right. And my party isn't bad. I just cleared INT S-4 this morning.
Researching a bit, you shouldn't need a tank till after the next trail boss. So probably around a month or two worth of time. For example, I did manage to pull a Cecil today, but I also could have used any of the three new characters and I had my free tickets sitting around. You're in the same position I am where any one of the three will do you well. Basically any 5* character will be a solid improvement too. So it's definitely worth pulling since you have about a 30% shot at getting a character from the boost and an even greater one for a useful character.
But you're also clearly not at a progression stop yet. If you're at S-Int. You can clear Europa, Demon Chimera, and anything else out currently, and should be fine for quite a while.
What's going to kill people later on is getting accustom to two factors.
1. The amount of Lapis and pulls we got now is an artificial increase to get you addicted. Pulling in this game is not cheap at all. 25$ for 11 and that is only a shot. So while we have people who have pulled 20-30 times in the past month. They'll have to get use to working for 5 pulls a month, or better to say, they'll have to make sure their lapis goes further. Training yourself now to wait for dailies, sales, etc will get you ready for the crunch. No different than waiting for the old dailies on Steam. The rewards for patience could end up being 2x as many pulls in the long run if they follow the Japanese route.
2. From what I read, the curve in this game is built around the top units giving a much easier time, but stuff is consider fair with the decent mid tier units too. Again, when the progression hits, you'll be in better shape than other F2P players.
It's no different than MMOs.
The good/bad news is, since the game is always changing too, you never know what is going to be up and what is going to be down. Lightning dropped from a 98 to 95 in the ratings on the JPN side. She is no longer the golden character it seems. While there seems to be no change to Chiz, Barts, CoD, ExDeath. Basically, it pays to have patience. When I was rerolling, the units I was told to aim for were CoD, ExDeath, Chiz, etc. I got tired of it since I never got any of them after 15-20 tries. Said fuck it and just stuck with my first account. Got Bartz after using my tickets on that one. Got Kefka after they gave out some more. Manage to luck out with Cecil and Art today with some more. I still have yet to actually get one of the characters I want, but I've manage to get some luck over time and get characters I need.