Non of that matters. When Sanji left WCI, he was fully confident in his abilities. Showing that nothing in WCI made him change his stance on wearing science. Especially something Germa made.
So the only logical conclusion is Kaido's boro breath made him change his mind. However Oda never showed that nor hinted at it. Which is why I am at a loss of words. How can he make such a big moment in a character's life have little set up. As nothing before post WCI counts as set up for Sanji's decision. Just background....
Maybe I could relieve some of your frustration, but I'm pretty sure Oda didn't intend for this to be a big moment or anything. He just wanted to establish the RS in Sanji's use, to set it up for future plot. He can't make Sanji reach his limits this early on, but at the same time he needed the RS to be introduced early on, again, for future plot. And because it's a set-up, it cannot be be a big moment. Only in the scenario where Sanji first went all out and then decided to use the RS would it instead culminate in said moment. But Oda clearly avoided such a scenario, one that most readers probably expected, and something Oda likely was aware of. Instead, he set up the whole thing with the SH needing to be anonymous, and the two Flying Six members going after an unknown Soba-seller, to introduce the RS in a non-desperate context(purpose of concealing identity).
The main reason, I believe, for bringing forth the RS this early and without a big, poignant "moment" is Oda's desire not only to set up the RS, but specifically the ability of
invisibility. If Sanji's first use of the RS had come after reaching his fighting limits, then that would have to be a the end/near the of the story. But Oda clearly wanted to give Sanji invisibility earlier, for him to use as the story progresses. That's basically it, I believe.
Oda has been itching to give Sanji this ability for a loooong time. Like how Oda set up for Zoro's Enma back on Thriller Bark, he didn't actually set up for Sanji's Raid Suit itself, but rather the ability of
invisibility. Sanji has been an "invisible man" since the beginning of the story. Even when he didn't possess invisibility he was still invisible to the enemy. Both regarding his tendency to take detours that end up saving the crew, making him "invisible", but also on his bounty posters was he "invisible".
As Sanji's true identity is finally revealed to the world, he gains literal invisibility. In a sense, it's a continuation of his character.
This tells us about how Oda fundamentally views the impact Luffy, Zoro and Sanji have. While the former two make difference on the battlefield, Sanji's impact extends beyond that, as we've seen on multiple occasions where he has steered the fate of the strawhats away from demise. The tool has always been his mind. Now add to that this new ability.
He's going to prove immensely dangerous to the enemy's organization and operations. They got no clue what's coming.