I'm about to blow your mind, are you ready?
Queueing for the highest level dungeon you can is more experience than using leveling roulette.
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^ this.
Other than that, the frontlines roulette doesn’t give a lot of tomes but gives good xp so it’s better used for leveling. MSQ roulette can give a lot of xp too if you get Prae but not so much if you get the trial.
If you need extra tomes, consider joining a hunt train: a full EW train of 12 A rank marks gives 240 tomes of causality and 120 Comedy+ 1200 poetics and nuts and some clusters (rng). If you catch a double train for EW and ShB, add another 120 tomes of causality + lots of poetics and nuts. A single S rank boss from EW gives you 80 tomes of causality, 30 comedy and a ShB one, 30 causality. Get a hunt cross worlds linkshell and/or join a hunt discord for the announcements.
Idk if you are not familiar with hunt trains, but just in case, you need to join a group whenever you see a hunt train announcement (PF groups always form then), move fast and make sure to hit each boss. A rank bosses can die in a few seconds if there are lots of people. It’s the party’s enmity and not dps that counts and why you see a lot of tanks and healers (I just spam range attack on tank: one hit generates enough enmity to give me full reward and the weak dps makes the boss die slower so as to allow everyone the chance to get their full reward). If you don’t join a group, you will get only a fraction of the rewards. There’s about 3-4 hunt trains per active server/day and you can switch servers to get on more trains when you are online.
Unfortunately the problem with this idea is that the tomestones are the carrot to get those who don't need the roulette for experience to run it. If they separated out a roulette that only gave experience, you would most likely see increased queue times because only those who needed it strictly for leveling would use it. What we have now works because we roulettes stay populated through having different types of rewards and thus offering different types of carrots to give players reasons to run them.
And yes, dungeons and trials and raids in and of themselves give non-experience rewards like tomestones when just manually queued into, so just review those in Duty Finder and you can run one specific one if you need to top off tomestones and your roulettes are used up.