Blue Mage makes for a great solo subjob for Dragoon, since it offers several spells with short cast and recast times as well as low MP costs for the sake of triggering Healing Breath. Not only that, it also offers a very good buff in the form of Cocoon, which is a 50% increase to your defense and against fodder mobs offers a noticeable decrease to the damage taken. However, a downside to Blue Mage as a sub job is that unlike Red Mage, none of the spells you're going to be using for Healing Breath trigger can be cast on yourself. Whereas /RDM has the self-target elemental Bar-spells that they can spam on themselves without attacking a mob, /BLU's common trigger spells (Foot Kick, Power Attack, etc) all target the mob, and thus aren't good to use when recovering from a battle. You can use Cocoon, Pollen, or Wild Carrot to trigger Healing Breath outside of battle, but Cocoon isn't something you can spam quickly, and Pollen or Wild Carrot might raise your HP level high enough to jump past the trigger level.
Most Red Mage spells that you'd be using as a subjob are fairly cheap though, at least the ones for early on around your level. If you take Red Mage to 49 for a viable solo sub through to 99 Dragoon, you may run into snags in regards to Phalanx, but that's about it, and at your current Dragoon level, you and a couple other people could do the Waughroon Shrine BCNM 40 for gil rewards as well as the aforementioned Phalanx. Dragoon is a good job for those, and there is enough gil rewards from that, that you'll likely not have much trouble getting assistance with it.
After Yuhtunga, you can probably both exp and skill up in Garlaige Citadel doing page 2 till about 56 or so, or do Crawlers' Nest till 57. Given the progression of mob levels and books, one could argue that you could stay in Crawlers' Nest a good deal longer after 57, jumping to page 4 all the way to level 70.
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