Keep the same settings as your first button. GTRR with "match current record" pulls up a found set of all records listed in an unfiltered portal, but makes the record clicked in the portal to be the current record so "match current record" is the setting to use.
This is because "match current record" matches the current layout record to the records in the portal's table. "Match found set" will match your layout's found set of records to the records in the related table and produce a found set of what could be many many more records and definitely not what you want here.
Just what I wanted, thank you!
I guess I didn't understand that by having my first button on the row itself, it was simultaneously clicking the record, which is the reason I was just getting the details for the single record.
GTRR is complex, something of a "camel" (a horse designed by a committee) and not really documented down to every last detail in FileMaker help.
I've taken my own stab at documenting it but I think it still leaves out a detail or two: The Complete Go To Related Record