There's obviously something missing here.
Assuming Solomon is right (big if), the new timeline is:
Feb 2019: Ukraine General Prosecutor re-opens Burisma investigation (or plans on re-opening)
April 21 2019: Ukraine Presidential Election
May 20 2019: New President takes office
July 15 2019: Trump stops ~$400m of aid to Ukraine
July 25 2019: Trump/Ukraine phone call
August 29 2019: New General Prosecutor for Ukraine takes office (replacing the guy that re-opened in Feb)
Couple possible explanations:
Trump didn't know that the investigation had already been opened (seems unlikely).
The investigation was closed before July 15, and now that there was a new President (and soon to be new Prosecutor) Trump figured he could get them to open the investigation for the third time.
If this were something that made Trump look good, and Trump knew about it, he'd be screaming it from the hilltops weeks ago. Now that it's been on fox news though, I bet he starts.
Except Ukraine was not informed of the funding being held when he made the phone call, thus there is no way it could have been interpreted as being used as leverage over them because they were not aware of it. He probably did know the investigation was opening again, and wanted to make it clear to the new president that not only would he not resist the investigation, he wanted it to happen.
The big picture you on the left are missing here is that this phone call was primarily designed as a trap, as made evident by his mentioning of Crowdstrike during the call. He is a brilliant tactician who understands the Democrat dynamic of accusing their opponents of the crimes they themselves are guilty of, and they would not be able to resist trying to cast this as foreign election interference. The Democrat party and the media took the bait hook, line, and sinker, and he used them all to blow the Biden corruption wide open. This is judo 101, using your enemy's momentum and force against them. Your unrelenting blind hysteria is only serving him now.