1) Greater Invisibility + Pass Without Trace. Telekinesis. Dimension Door x 2. Every situation has an easier solution, except ones in which only Time Stop would work (which are simply contrived).
2.) Time Stop. DBF. Blink - oops, time's up.That's the thing: even buffing isn't really worth it because you don't know until you cast how long you'll actually have. Also, some stuff like Armor of Agathys should probably have been cast an hour ago. But I agree you've more or less come up with the only possible usage: surprise insurance. If a wizard was completely unbuffed and ambushed, this would allow them to get at least two buffs going for the price of one action and a 9th level spell slot. Although - Contingency.
But it's expensive and only works on one 5th level or lower spell. But still: Contingency + Dimension Door is probably still a better use of your resources.
If we're talking about fixing Time Stop, I'd start by giving it a fixed duration. 5 rounds seems fine. That actually gives you something to work with, and something you can plan for. Making it also work as a reaction also doesn't seem overpowered (if you made it only work as a reaction, that would cement it as a purely defensive spell). That allows you one "don't get hit for free" power, which is really, really amazing, especially vs. some late-game save-or-die stuff. Imagine Time Stop as a sort of counter to something like Meteor Swarm - you simply got out of the way.
Delayed Blast Fireball. 12d6 + 10d6 = big badda boom. DBF has to go off within 1 minute and each 6 seconds adds another d6 to the damage, so any duration of Time Stop longer than one minute means you could cast it to its maximum effectiveness. It doesn't matter how long the Time Stop was supposed to go: the DBF would explode, ending your Time Stop, but then you've probably done all you wanted to do already.
On the other hand, nearly doubling the effectiveness of a single level 7 AoE damage spell DOES sound like the type of thing you'd want to use a 9th level spell slot for.![]()
Instead of casting 40d6 meter swarm?