as far as i know, there’s no date or activity id captured at the moment an observation becomes research grade. so there’s no clean / perfect (+ efficient) way to get observations sorted by when they became research grade.
that said, the closest approximation would be something like this, i think: https://jumear.github.io/stirfry/iNatAPIv1_observations.html?user_id=mrtnlowr&quality_grade=research&order_by=updated_at&options=idextra.
that gives you your RG observations, ordered by update date (descending). it includes a few extra columns that tell you how many identifications have been made, and whether the IDs are at different taxa.
so assuming that the last update to an observation is likely an ID (and so sorting by update date is a proxy for sorting by last identification date), and knowing that it usually takes at least 2 ids at the same species to reach RG, then if you go down the list of results and find the first observation with values in the ID columns that fit that criteria, that should give you your most recent observation to have become RG.
looking at the results, the first several appear to be observations where folks are agreeing with already-RG observations (adding additional IDs beyond the needed 2 for RG). the first one that has just 2 ids at the observation taxon without any other ids is https://www.inaturalist.org/observations/68000134:
(that said, if someone makes a second identification to make an observation RG, and then another person comes in immediately after and makes a third agreeing identification, there’s not a great way to see that that second identification was made at roughly the same time as the third. so that’s a limitation with this approach.)
…
this will give you notifications of activity on your observations, but it won’t necessarily tell you when observations have become RG. that said, if you’re just looking for old notifications, you can use the thing discussed here: https://forum.inaturalist.org/t/is-there-an-easy-way-to-open-all-notifications/25913/35. (in the filter parameters section, set viewed=[blank] and observed by=you.)