You probablycould make this whole ability a lot easier on the rulesby making it a triggered ability that say "Whenever you cast a spell, you may pay" etc.

But even if you had some top-notch reason to make this a static ability, the "If you do" probably wants to be a "When you do" pretty hard. OTherwise there are a lot of open questions regarding the timing of that ability.

I don't know what the original was exactly, but I would imagine SI might have meant to not have the ability grant an ability to those spells but rather just straight-up make it directly trigger from those.

My recommendation for the wording:

First strike

If ~ would leave the battlefield, instead shuffle it into its owner's library.

Whenever you cast a spell, you may pay . If you do, destroy all creatures and planeswalkers.

Wait, what does that last ability on the current card do? So this wouldn't have the ability, but this would grant that ability to all creatures and such you cast (also I don't think it would do anything with nonpermanent cards)?