This method worked before I upgraded to Android 9, now Google Play Services will only downgrade to 12.8, and I can no longer install an 12.6 because it's older. Is there any way around this yet?

disable google play

Sorry to be superfluous... so this works on something like a bootloader locked S7 Edge? I can get root, but not Magisk because that requires unlocked bootloader =(

UPDATE: I tried downloading a couple of the Play SErvices apks, but all just say "Cannot install, package is corrupt" I've downloaded multiple times and tried ones that are similar but don't match the "040408-xxxxxxx" like the 040400 one. Any ideas?

I'm on Beta 2.1 and my Webview version is the same as yours, but the very latest version seems to be 91.0.4472.101 so we are very close. My Google Play Services is on 21.21.57, but I am on the beta channel for that app so not sure if the version number is comparable to yours. Is there a newer version of standard Play Services above 21.18.16 ?

Point is, these versions are so close to current that I'm not sure that the difference would even trigger an update, so are you sure you (or we) are really 'stuck' on anything? There simply may not be a new(er) enough version to trigger the update process(?)