How do you define (meaningful) choice?

[TheDustyOne]

If there is one right choice and everything else is wrong, then there is no choice - meaningful or otherwise.

I disagree, the choice is meaningful, just not deep.

Let's say we take your Broil IV and Ruin II example for a minute here. When we don't need to move, Broil IV is obviously the right answer, but the choice to use Ruin II is still there, even if it's the wrong choice in that situation. The choice to use Broil IV over Ruin II is meaningful because we know the meaning behind why we're using Broil; that it does more damage than Ruin. Again, not exactly deep, and in a target dummy situation it makes Ruin II button bloat, but there's still meaning as to why someone is using Broil over Ruin.

Contrast that with someone deciding to use Titan over Garuda against a target dummy after their demi phase. They both do (roughly) the same damage and (roughly) the same mobility outside Slipstream. There's 2 choices, but it's not very meaningful, just choose whatever and your DPS isn't likely to suffer. Ruin II may be more situational and we ideally don't want to use it at all, but the choice to use it is ultimately more meaningful.

Also I know that when optimizing SMN, whether to use Titan or Garuda first can depend on party comp, but I'm talking in a general sense, the DPS gains are ultimately quite small.

Meaningful gameplay is about having consequences for your choices, depth is having lots of viable choices that are also meaningful.