Okay, increasing the collection of statistics seems to have helped. I used `ALTER TABLE report ALTER COLUMN reporter_id SET STATISTICS 10000` and now queries which previously didn't finish at all now finish in < 1 ms.
The following gave me the hint: “The amount of information stored in `pg_statistic` by `ANALYZE`, in particular the maximum number of entries in the `most_common_vals` and `histogram_bounds` arrays for each column, can be set on a column-by-column basis using the `ALTER TABLE SET STATISTICS` command, or globally by setting the default_statistics_target configuration variable. The default limit is presently 100 entries. *Raising the limit might allow more accurate planner estimates to be made, particularly for columns with irregular data distributions*, at the price of consuming more space in `pg_statistic` and slightly more time to compute the estimates. Conversely, a lower limit might be sufficient for columns with simple data distributions.” — https://www.postgresql.org/docs/current/planner-stats.html >
