I'm learning the "HSK 2", "HSK 3", and three 成语 lists. What I really want is for the SRS to give me 50% HSK 2 (so that it gets done) 25% HSK 3 (to challenge me) and 25% 成语 to keep me entertained.

AFAIK there is no way to ensure this?

Nope. You would have to manually add stuff while pausing and unpausing the lists you want in the ratios you desire.

Although perhaps useful, I would dread the idea of making an intuitive interface for what you describe.

There is already a way to regulate the rate at which new characters are added overall; one would just apply those each list. As far as I know this would not interfere with SRS. I have to say though, that it's not clear how many people want such fine-grained control over their study.

Pause/unpause is probably fine, but that feature seems to have gone? All I see on the practice page is "cram" and "remove" buttons?

For an intuitive interface you could use sliders next to all of the lists (perhaps enabled by a check box in the options menu to reduce clutter for the average user), where the sum of all of the sliders equals 100%.

阿福 - you can pause and stop on the actual page where you write characters, by hitting the active lists button in the top right corner.

Found it - thanks, Byzanti.