Rebuking this with
this:
Zephyro suddenly holding back here is a mischaracterisation as 'He is obsessed with the final moment of annihilation - the beauty of absolute erasure.' - He will do what it takes to achieve these things, regardless of if it's his FP or not. It's in character for him to annihilate everything, other characters put him as the most merciful Lord Ravager because he gives a death without pain.
This establishes he should be trying to completely overwhelm his opponents. Whether it's 1-shotting or overwhelming.
Now, why would he hold back against Welt when he is trying to annihilate Welt? A simple answer would be that Welt doesn't require Zephyro's full power.
If you believe the above to be true, you need to explain away how their attacks, the black hole and white hole, clash and cause larger destruction than what Zephyro is capable of alone.
No, you can't explain this away with the 'black hole and the white hole' line.
I believe that the detonation of the two opposing forces destroying the galaxy can't be attributed to the clashing natures of the black hole and white hole alone - In this case, even a small black hole and white hole would mimic this feat as their natures still oppose each other. This obviously doesn't make sense. It's more likely that the two attacks combined had enough energy to destroy the cosmos, their opposing nature just made the two detonate and destroy the universe.
The voiceover even says the black holes 'devour each other' - More of a clue that the two combined their pools of energy and detonated, resulting in the destruction of the cosmos.
You also need to explain away why Zephyro wouldn't start using his FP against Welt the moment the two clash. Zephyro is 'obsessed with the final moment of annihilation' and 'the beauty of absolute erasure'. Why shouldn't he want to absolutely incinerate Welt? The Astral Express in this timeline are threats to the Lord Ravagers, they've already defeated Phantylia. Why would Zephyro hold back against them when they're legitimate threats to the Ravager's goals at this point?
These aren't statements for someone who would hold back because they enjoy being contested. Yes, Zephyro was contested here. The black hole and white hole were still, then the white hole overtook the black hole.
If you can't explain away all the inconsistencies in Zephyro's power and character, this combined destruction feat implies Welt's power IS significant in the clash, as from what we know of Zephyro, he can't destroy the entire universe alone.
Long story short I think we should come together and make a discussion rule about this.