Hi. I remember talking about some of this with you before so I'll try not to rehash too many old ideas but I'm also happy to because I never felt like our persons were under attack so /wave lol.
For the Titania extreme example I'd say that it would indeed require clever planning of when and where abilities are obtained, but also that I don't think it needs to use extreme primals to get it done. Like for the most part you can use hard mode primals as well, which is far more farmable. Also on a personal aside note I don't really love that blue mage can learn primal spells since I think that should be what summoner is doing (bane be gone, cast eruption instead, etc, etc), but that's just a personal feeling.
For a loose example we could have every 10 levels the blue mage has a page in their blue mage book (for DF at least, not discussing the advanced job concept at the moment). Blue Mage levels via their book.
So you might think of a level 25 dungeon, and because I want to be lazy and make an easy example lets talk about esuna lol (don't think about what our jobs currently are, this is a new healer job). This dungeon requires esuna in order to go smoothly. You earn it at level 24 by doing a job quest (something SE has moved away from because they dont want players running around without their kit lol). I join and I'm level capped to 26 but still don't have esuna, annoying right? I think everyone agrees.
So in this blue mage book example, esuna-b is their level 24 skill. They're currently level 22. They go out to the monster that gives it. They learn it. They're now level 24. They couldn't join a dungeon without it and the DF doesn't even need to check their spell book because it's a level issue. We could make that concept granular even like how our hunting logs worked (if people recall that log that just sort of collects dust at the moment lol)- in order to complete your log you need x y z tasks. So to reach level 25 could be spell "x y z", this allowing blue mage some freedom in the order they learn spells in that bracket but still restricting them into a space that meets DF's expectations. Of course if you join a level 23 dungeon people shouldn't expect you have your level 24 spells, and this is already true- sometimes you gain a level in a dungeon and gain a new spell in said dungeon and I think that's fine and not fair to have issue with (not saying you did, just saying "some" missing skills depending on why is okay).
Under the advanced job concept, where blue mage has a huge book of spells, and some of the spells are gilded (surrounded by a gold marker) to notate those are DF spells. Where Blue Mage can become unchained for any non-current content or chained for current. I think it's a bit easier even as we could keep the same fast open world leveling and just tell blue mages "if you want to join the content you have to have the spells of that dungeon's level and lower". So it just becomes a sort of shopping list. Of course if one of the spells is like "Bahamut Ultimate" you'd have a problem.. but I'd say.. don't do that lol. Also these gilded spells could have adjusted learning rates (start much higher). Because blue mage would level much faster I think the learning would offset that and make it "fairly" fair that blue mage levels the fastest.
Finally on weird spells and versatility I agree with Duelle that it's not necessarily that they're able to do all roles well or be a jack of all trades.. things like that and it's more just they've access to abnormal ideas. I think for many of these abnormal spells you can make them work in spirit even if they're not perfect clones. Say for example bad breath will really mess up trash packs for a short period of time but the main feature of bad breath, the feature that exists into boss fights even, is the poison and defense down debuff (like a less powerful trick attack debuff). Novelty in that blue mage could pause an entire pack for a few seconds, but function for when the novelty must fail (bosses, could even have the bosses just ignore all the status afflictions but the DoT / def down, so it's easier to balance than "oh I forgot to make that boss immune to paralyze.... now blue mages are abusing it"). For death you could have something incredible like on a /huge/ cooldown you can outright kill one trash monster, perhaps for a little play and chance it either kills them outright or deals like 75% hp. Once cast death turns to something like mortal ray or death ray, a high powered damaging spell that is combined in some way in the rotation of spells. Each time you cast death ray it lowers death's cooldown. Death automatically changes to death ray when you enter boss rooms (meaning there is no way to cast it on a boss).
I do feel you're presenting issues SE would have to thoughtfully consider, I just don't agree they're issues that cannot be solved or at least cannot be presented in some fascinating way without having to kill the DF portion of blue mage. Like I think it'd be interesting if blue mage leveled fast but had to go get their own spells, or if they unlocked new pages of the book as they leveled, or whatever other idea that fits in there but still allows DF. Something like how I was fine I had to go pilgrimage for primals in FFXI as it was flavor to the job, or in WoW where you used to have to take care of your pet and pet choice mattered more (it's a bit annoying some days but I think the pet feeding mechanic was awesome... loved that I had to pick up fishing and sometimes just stop what I'm doing and relax a bit to make my pet happy again). It's why I guess I was thinking advanced jobs could be fun, the idea they bring something different to the table we're all used to sitting at- they require you to do something a bit abnormal.. that they level different or require a different feedback loop.. just changes on some core principles that you don't normally expect to change- I think that's kind of exciting. I don't love limited jobs though because I think there is an incredible potential for disappointment (like people who wanted to main blue mage in DF, like I did). I'll admit there can be some fun concepts still under limited but I still think the potential for disappointment is grand (like I'd probably enjoy blue mage more if it was a solo god, as we talked about before).