To avoid that the number of pending reviews gets out of control add a limit to the number of pages that can be protected with forced review at any time. This number should probably be dynamic and reflect either the number of reviwers or the number of pending reviews at any time.
This will use approx the same mechanism as https://bugzilla.wikimedia.org/show_bug.cgi?id=70456
Version: unspecified
Severity: enhancement
It would be easier to disable the extension when it gets out of control, or to let reviews expire on a page when they have unreviewed edits older than x days (30? 100?).
A limit on number of days a revision can be unreviewed is simply an algorithmic variation.
I'm not sure what you mean by "simply an algorithmic variation" and whether you're saying that it's a good or bad thing, but the whole point of FlaggedRevs is to have manual flagging of things, so at the very least the flags need to be understandable.
I think an expiry would be easier to understand than a potentially arbitrary limit on the total number, but I don't have a strong opinion on what would work.