First and foremost. Please don't do this if you can avoid it. Just back away from the mouse and keyboard and rethink everything you're doing. I personally hate myself that my own work has led to situations like this, but I recognize that sometimes these kinds of applications expand beyond your control.
So, for those of you who are trapped and have to come up w/ a solution, I've laid out some strategies for dealing with large Lists in SharePoint Online (let's say 5,001 - 500,000 items) and some of the issues you will encounter when using Power Apps to access them.
NOTE: As of 2021/10/5 my account has been throttled and my list import into PowerApps from Excel limited to 15k rows. Working w/ MS to understand if they have a better answer than the non-answers we have today. The key to avoid this is you can do everything I outline below, just don't open 3 VMs and start doing the same stuff under the same account in each. :D
