If you do take HE from park to park, watch for the station to open. They will tell you that the station opens at a set time and not a moment earlier. But I have seen them low key start admitting people early. If you are observant, you can beat the rush to the train. The one time we did this, we got on the first train of the day which was pretty awesome.
They'll let people line up outside prior to 9 (as they cannot stop people from waiting outside), but essentially will not start letting into the actual building at most a couple of minutes prior to 9am. As the train is intended to leave at 9am (if there are no delays). If an attraction doesn't run until 9am then there is no reason for people being in the queue. If they did that for HE they might as well do it for all the other attractions. I think that they prevent people from waiting in the queue also to prevent confusion, or misconception. You can tell plenty of people that it won't be taking off til 9am, but once they're inside they might still get impatient, forget, or still expect to do so, but the message will be clear if they cannot literally enter the building. Some people I think would just believe if they get inside they'll get on the ride early regardless even if they stated it wouldn't open til 9am. Especially considering how many people I've watched just go on and on and on with the TM out front needing to explain their case as to why it isn't running until 9am.
The last time I was there this Sunday, there was already quite a few people lined up outside and it was only 8:30am. Prior to that, I had waited outside for the "first" train a couple of weeks prior. I think I had a couple of dozen people in front of me, so still enough to be on the first train, but by the time everyone flooded into the building and went there separate ways into scanning tickets I didn't get onto the first train, but however did get onto the second one which is still good.
Also, while I know the OP said they're not interested in rope dropping for Hagrid's, but I want to let others know this isn't the best route for rope drop for Hagrid's. As people at the IoA gate get to line up 15-20 minutes prior to official opening at Hagrid's and not only the train leaves at 9 (if not delayed), but it's also a 5 minute ride. However, it is a good way to get over there if you plan to skip Hagrid's and move onto the other things in Hogsmeade in general.
I do want to say that in general an official opening time is always a few minutes early. It's the same for both theme parks. Even when they have people who are holding off people to enter the rest of the park (as EE only allows certain attractions) they usually will let people through a few minutes prior to the official opening time, but only by a few.