I feel you, I do, even if I don't agree with some of your personal takes. Ninja feels like it's being tugged in two distinct directions right now, and it just feels like a mess as a result.
I'd feel like the levelling argument would hold more weight if not for two things:
1. Levelling is temporary, endgame is forever. While it's important to make sure the levelling process isn't outright awful, I strongly disagree with making sacrifices on the part of Level 90 content for the sake of making a temporary situation better. You spend so much more time at level cap than you do at any other point during the game. Levelling is faster than it's ever been, too.
2. Other jobs suffer worse! Dancer and Bard are even more reliant than NIN on party members feeding into buffs to make the most of their kit, so should we gut them as well? What about healers, with their incredibly low personal DPS? Sure, they can heal themselves, but doing any kind of solo duty takes twice as long.
I do really agree that levelling Ninja doesn't feel super great, but I just can't agree with gimping Trick and giving more personal damage for the sake of levelling. Rather, we should tune solo content such that it can be completed by any job.
I do see what you're saying about Ninja being either very strong or very weak, but even in 6.05, where Ninja was arguably at its weakest in a good long while, it was still very possible to complete content with the job. I cleared P4S during 6.05 on Ninja; as did many others. While I do empathise with not feeling like your job is strong or worthwhile if your numbers hit a little lower than other jobs in your role, I don't think Ninja has in recent times had the issue of being so weak it was excluded from parties. Of course, this is me speaking anecdotally, maybe others have had different experiences.
We've already kind of gone down the track of 'Ninja's personal damage is too weak, it's too reliant on others for DPS' with the move from 10% DMG up -> 5% DMG up in Shadowbringers, which is a pretty substantial nerf to rDPS! It's very difficult to argue making a personal buff much weaker; 5% is simultaneously not a lot of damage and also a huge amount of damage.
I guess my main question is, where do we draw the line? If we follow this train of thought to its logical conclusion, every job becomes selfish and party buffs cease to exist. I don't think anybody genuinely wants this, nor do I think we'd ever actually get to that point, because (at least in my opinion) party buffs are fun! It feels good to buff and be buffed. It's rewarding when you play into it. But -- the fact remains that if you continue to gnaw away at existing buffs, jobs become more and more selfish over time.
The biggest loss, however, is that NIN's party buff is no longer on the 1-minute cooldown, which is just heartbreaking for so many reasons. For many reasons, including what you pointed out.
I do disagree that moving Trick to a personal buff opens up more flexibility in design. You're still pressing the same buttons, and Trick is still a damage up ability, it just only affects you now. Trick becoming personal doesn't affect how many abilities you press inside of Trick. Rather, Mug becoming a party buff does, because it prevents you from pooling Ninki and means you have one fewer oGCD to hit inside Trick. While this does technically more oGCD weaving windows, I'd argue it's a regression because what kind of builder-spender job isn't able to build for burst? Ninja is uniquely punished at the moment for entering a two-minute burst window; you're in most situations forced to use a Bhava before you get your buffs up, which feels like such a waste. Removes the itty little bit of skill expression of drifting Mug into Trick, too.
I don't know, I just... mm. The solo experience shouldn't suck, but it just seems a little backwards to me to balance an MMO -- a multiplayer-focused game -- on a solo player experience. Make it not suck, absolutely! Make it reasonable and clearable. But -- shit, Ninja was one of three big buffing classes in the game. Now we just have Dancer and Bard as big buffers. Sure, many jobs have two-minute buffs (12/19, by my count), but when everyone has a two-minute buff, it's the same thing as nobody having a two-minute buff...