Don't over-think this or seek actionable answers from anecdotes which will be all over the place based on experiences which will be all over the place.

If you don't like the room that's allocated to you when you check in on line then nothing stops you going to the front desk and asking if an upgrade has become available in the meantime. I did this at the JW Hong Kong a few years ago and got my suite in person after checking in on line and being allocated some lesser room.

I check in on line just to tell hotels approximately when I will be arriving as I think this is helpful information for them and if they're in the habit of upgrading elites then maybe they will allocate you an upgraded room at this point before you arrive. Indeed, my favourite hotel has already upgraded me on the app some five days before I arrive this coming Friday (and way before I am given the chance to check in on line), for example.

I don't think there's a way of gaming the online check in process to steer it in your direction. Upgrades can be applied by the hotel at any time before or after you check in online or in person and it's dependent on the hotel, brand, and region more than any online check in. As I say, if you don't like what they've allocated you then you can always ask when you physically arrive.