It looks like there are over 2,000 plushies to collect, but the pizes I see only go up to 500. Are there more prizes once you complete the 500 mark?

Not yet. Beanbags stop at 500 too

bummer

When I joined Subeta three and a half years ago, 500 was about half of all of them. Many of the remaining ones over 500 were rare or donation presents, so it would've been a huge challenge to get them all back then.

It is still a huge challenge to get the all. The ones that were rare and donation presents back then are still rare and donation presents. :P

You still get achievements for collecting more. That's an incentive for me, not sure if it would be to you or not.