Solo content is actually pretty straightforward. Outside of your standard UEs, advents and raids, you add only AQ5 + 6 to the equation, and you add tower to the equation. For all content, hrags + guardian raids are the extra things to look out for. In short, everything, unless they play this game casually to begin with.
I wouldn't think of it as optimising to clear a specific content. Rather, I frame it as optimising a team in general, and that means accounting for the worst thing that can happen. You have probably seen it in some fire setups, people are choosing to go yolo no debuffs simply because wirag gives the finger to debuffs. That's building around wirag yes, but at the same time since wirag is the hardest content to exist, being able to beat it (solo) usually means you steamroll other wind content no issue. Similarly, optimising to clear tower 15F presents a similar case where once you get to the point of being able to clear it, it's pretty safe to say you can clear most of other content with ease, because tower so far is the hardest solo content you can find.2. How does optimizing to clear this content improve a player's strength long-term?
And do note I specified 15F, because tower is quite frankly not just about the difficulty, but also about your micromanaging of girls to properly clear lower floors sandbagged for maximum gain, which is probably what nonsensei likes the most about tower. Basically, I take the hardest part of tower to prove my point.
The meta actually doesn't change that much in terms of playstyle. Except you know, all that shit I've been giving on debuffs (which I'll further address in point 5). There isn't a specific answer if you were to take this from the perspective of fighting the boss since they have so many designs (a point I'll get to in point 7), so you rely determining your playstyle by element, since anyway that's exactly relying on what you have. A full breakdown of element playstyle is something I can't do myself considering I didn't invest much time playing in the DMM version yet however, so the only sources you have at the moment is the DMM wiki, as well as a few helpful DMM players that may be lurking around.3. What are the main focuses in the meta? Spike damage to get through a particular phase? Sustained damage to end a long fight more quickly (e.g., before a deathblow type of ability)? Certain defensive abilities? Are these focuses general-purpose, or would they be expected to become obsolete or marginalized in future content?
In content where its effective, these will still be very, very applicable. Heal will still be used in long-lasting content that doesn't throw things at you designed to not be survived. Dispels still work in content where buffs are periodic, and really game changing. Cleanses still work where debuff spam is heavy, or potent. When these fail however, you can pretty much look at dps as the only other option.4. How valuable are healing, cleanse, and dispel and other general utility skills (from kamihime)?
To answer the question first, AQ6, tower, dummy event, and the future hrag raids (and guardians), basically future mainstay content. And a few oddballs like Icarus GO. How much? Not completely immune, but basically if you were to bring a loadout of 50% atk and def break... you'll probably land like, 10% atk and def break each on average. Can you actually hit the debuff cap? Certainly, why not, it's RNG and RNG sometimes favours you. Is it practical? I like to say that if there's a 20% chance of it succeeding, it's 70% too low. That's not accounting for the content that doesn't resist, but instead punishes you for using debuff skills (e.g. Barong, both her raid and GO form).5. How frequent is high debuff resistance?
So that is quite a sizeable portion of mainstay future content. When I say debuff is going to be off meta, this is what I mean. Debuff in current mainstay content? Sure, why not, no harm. Debuff in future content? You're gonna be selling your soul to the devil for only that to be reliable. This needs emphasis: I am not saying fuck debuffs as in don't use them. I am saying you need debuffs + something else that is really good.
That... depends on how you define an average player. An average average player (meaning including the casuals) pretty much won't, and will struggle with low cap damage skills like Susanoo and SSArty, because let's face it, it's directly proportional to the time you invested in the grid. An average player that calls himself good however... well actually I haven't done the maths, so I won't comment.6. How often will a typical player have to worry about hitting damage caps?
Elemental advantage is a boon for reasons we have discussed before: doing more damage and doing less damage. An easy example to fall on is wirag... I'm sure you realise the pain of trying to clear that thing off element. Extend that pain to hrags, which you certainly won't be doing alone, and where farming is way less painful if you get MVP (drop rates of hrags are actually atrocious otherwise). A player running an advantage element team has a higher chance of getting the MVP over the player who doesn't for 2 reasons:7. Presumably elemental advantage is a current focus. How does this affect one's ability to clear content and one's efficiency at earning rewards?
a. Snip
b. In cases of some elements raids, the advantage elements against these raids also have team designs that counter the raid style hard.