While I can't speak for registration for the past two years (2019, 2020) in previous years that I've done PHM, the 10K and Challenge tend to sell out the quickest - within a couple of hours of registration opening. Then the half marathon, and then the 5K. Some years, I've found either the 5K or HM still open within a few months of race date, and some years (usually anniversary years) all of the races sell out within a week of registration opening.
Your best bet is to be ready on the day of registration. This means having your active.com account already set up and linked with your credit card information. I usually have the browser open on both my laptop and my phone since I'm often registering for more than just myself. Be patient, the site crashes often.
Next year is going to be a bit unpredictable, though, since many of the Star Wars runners have been pushed to next year. If I had registered for the Star Wars race weekend and had to reschedule, PHM weekend would 100% be my next option since we don't know how runDisney will be functioning this Fall for Wine & Dine, and I'm not big on the WDW Marathon weekend in January (that one might see a lot of folks being pushed that way instead, especially since 2021 is Disney World's 50th Anniversary, even though that's technically in October). We'll see, though. It's still a little too early to tell at this point.
Plan for the worst, hope for the best.