When can my opponent respond when I go to end step?

I'm building a commander deck with [[Baylen, the Haymaker]] and while I was brainstorming wincons, this question popped in my head. Say I'm in my second main phase and I use [[Second Harvest]] to get a truly absurd amount of tokens. It resolves then I tap all of them for mana and play [[Throne of the God-Pharaoh]]. It resolves again.

Now, my opponent has a [[Goblin Welder]] in the field, but can't respond because the stack is empty for now, so I have priority after the throne is in the field. When I declare I'm going to end step, that's when they can respond.

My question is when exactly are they able to respond? Are they able to tap [[Goblin Welder]] before I go into end step or after I'm already in end step? Because if it's before, I still have all my mana, meaning I can cast something like [[Heroic Intervention]] to save my wincon, but if it's after I enter my end step, my mana will be fizzled. Basically, do my opponents get priority when I declare I'm going into end step or when I enter end step?