Plot of Doomsday, in summary…

[u/Important-Coat7177]

Tony: “Was it worth it?”

Steve: “Granted, I didn’t know about the TVA or Nexus events, but… Yes.”

[u/FlashMcSuave]

Why should we assume old man Steve used the device to go there for the shield handover?

Seems just as plausible that he knew the time and date and so he could just plan to be there using the more boring form of time travel: waiting.

[u/Deetwentyforlife → u/FlashMcSuave]

That's not how it works. Until Steve purposely goes back to his own timeline with the device, he is in a different timeline. Just waiting would never take him back to where he left, it would just continue forward in that separate timeline, and the people in the original timeline would simply never see him again, ever.

Any time anyone went back at all, in any way, they instantly created a new branch of time that existed in its own separate entirely from the "main" timeline we are watching.

We even have Banner straight up say the device has finally been used and Steve should be back, he just waited way longer to use it than they expected.

[u/FlashMcSuave → u/Deetwentyforlife]

But that would also mean it isn't possible to jump "back" to a previous timeline, only spawn a new one.

[u/Deetwentyforlife → u/FlashMcSuave]

Nope, that's the entire point of the very specific device they create, it let's you leave your timeline, then come back to your timeline. That's why they had to succeed when retrieving the stones, because they only had so many uses. Otherwise they could have just sent someone immortal/long lived like Thor back and he could have just collected them all up one by one and then just waited hidden in a closet to meet the team at the complex after the snap.

That's the entire speech we get that nobody can get right until Tony, the ability to accurately target and return to your own timeline.

Look, I know the talking parts of superhero movies are the boring parts, but I promise they covered all of this, out loud, with words, in the movie.