Is there a way to search for a list of mature cards that were reviewed today? or that were reviewed within the past N days? The problem with ivl>=21 is that (and correct me if I'm wrong) it will evaluate whether the interval is >= 21 days right now, as opposed to whether the interval was >= 21 days when it came up for review. I don't want to include cards in the search that became mature after they were last reviewed... only those that were already mature. Is there a way to find these?
#Searching for mature cards that were reviewed recently
minor whale
finite gale
About "reviewed today", you can use the rated filter (https://docs.ankiweb.net/searching.html#answered).
However you have no access to the previous interval that way. Add-ons would have access to that kind of data.