I'm soon hitting the limit on favorites; what's the best way to move some large chunks (say everything matching a tag) to a private pool and delete the posts from favorites?

Man, you're fucked.
Maybe with scripts are some shit you may be able to speed things up (idk nothing in that field sorry).
But else you'll need to add them to a pool and remove them from favourites one by one. Good luck.

Thanks for the reply - good to know at least that I didn't miss anything. Looks like a scripting I will go!

1) Go to settings/my profile. On the url above copy the id number.

2) Go to your new pool and click import. On the search bar type the following, fav:id_number_goes_here.

3) Remove posts you don't want and then scroll all the way down. Click import.

You are done.

By the way. You can use the search bar in the import screen the same way you search normally. Thus, if you want to let's say import only general images from your favourites then you should type, fav:id_number_goes_here rating:general.