This post contains the data of two runs of the query. the first with explain analyze. The second run is with explain buffers. Between the runs, an explicit Vacuum Analyze was done on the table.
Observations We tried removing the index on the field is_grc_002. That did not have a big impact.
To benefit from HOT update, you need both spare room in the table, and to not have an index on the updated column.
So just dropping the index is probably not enough for a full-table update as you don't have the spare room. You also have to populate the table with a lower fillfactor, as has already been noted, as well as dropping the index.
Is this update a one-time thing, or does the application do it on a regular basis?