I set a 20 character password using Find My Device, but the lock screen only accepts 16
For other people with this problem - I found out that sometimes the manufacturer of the phone can unlock it or even back-up your data before a factory reset, if you go to one of their service centers.
In my case, my phone is a Samsung, but my local service center still couldn't do anything because my specific model number wasn't supported. They said they support the "popular" models like the Galaxy S series or the Note series.

So the first step would be to contact the manufacturer and see if they provide repair services for your phone. Otherwise, after this past week of research, it seems the only solution is to factory reset and wipe your data.