When it comes to fast progression, DPS are, indeed, the primary focus for all upgrades. This is due to us technically being right on the ilvl cusp, which makes enrage timers extremely tight. Additionally, unlike WoW and most other MMOs, FFXIV healers and tanks have an absolute bevy of tools at their disposal to heal or mitigate damage, respectively. Another factor is how scripted our encounter design is. You will always know precisely when high spikes of damage will occur—thereby allowing for planned mitigation. Perhaps the biggest reason why healers are very rarely the focus on initial upgrades is because they're essentially pseudo-DPS in this game. What most midcore and above groups tend to do is have their healers overheal/shield everything at the expensive of their DPS and remove the excess accordingly. For example, my group hyper-mitigated the Flare explosions in E2 at first. We wound up moving things around to primarily focus on the second set as the first didn't need it.

Please do bear in mind, the above only applies to statics. And not every static will prioritize this way. In fact, a lot may do a more fair loot distribution. It is something you will need to discuss with your group. If you're just pugging though, then roll on everything. You have just as much right to upgrades as anyone else in that scenario. So don't feel hesitant the DPS should go first.

Regarding Tenacity. It's... kind of meh for want of a better word. While it does provide upwards of 5% passive mitigation, it takes a massive amount—3,000, I believe. This is such a negligible amount compared to the damage you'll lose by sacrificing Critical Hit, Determination or Direct Hit that it simply isn't a worthwhile trade—especially when you consider your healers will be overhealing/shielding more than likely necessary to begin with. That being said, Tenacity isn't outright bad per se. Just lackluster. Put it this way, if you're low on funds and Tenacity materia is dirt cheap (it usually is), you're better off melding it than nothing at all. If you can afford it though, damage based materia will always be superior. When it comes to what melds, focus on Comfy Skill Speed > Direct Hit > Critical Hit > Determination. Warrior is the sole exception, and should ignore Direct Hit entirely.

If you want more information on melding and such, I highly suggest joining The Balance discord. They have everything from Best in Slot builds (some still being worked out) and just general guides on how to optimize and better perform on your job.

Hope this helps.