You report this as a bug against 15.3, did it used to perform better in a previous version or is this the only version you tried it in?
Your plans are unreadable. We are people here, not machines; you should use FORMAT text, not FORMAT json, and post them in a way that preserves newline and indentations. Email is bad at that, so maybe share as links to https://explain.depesz.com/ in addition to or instead of embedded text. And this doesn't seem to be a bug, just an infamous performance problem, so maybe use the pgsql-performance list instead.
You could likely get a big improvement by having an index on (event_type_id, date). It would still be using the wrong (suboptimal) index, but it would be a lot less wrong, as it could rule out the disqualified event_type_id without needing to inspect each one. It is also likely to be "good enough" over a wide range of parameters, unlike Tom's suggestion which would probably be very good for this particular parameterization, but might not be good for a different one with a much more popular city_id.
