Do you add singular and plural version of keywords (if they are relevant)?
Only Shutterstock is deleting the plurals automatically. Does it mean that all the others accept (and need) both of them?

Put plurals in as a default.  If the site strips them out (like SS) thats ok

Other sites need plurals to find the images.

Yes, the thing with using identical metadata for all agencies is that you only optimize for a few. Sure, it's much more convenient to use multisubmission services and/or to fill the metadata in Lightroom, but you won't optimize search exposure everywhere.

In the end, maybe you have time for more files so it balances out, but just looking at it from a search perspective, it's not ideal.