I've tracked it for a few years with whatever the latest samsung watch is, and it usually says it's good to excellent.

My total sleep time is about 6 hours. deep sleep is about 1 hour and 30 min. rem sleep is about 1 hour and 20 min.

I usually go to bed around 11pm and wake up at 6 am (give or take, I'm married)

It does however warn me about bad sleep if I've been drinking, have a fever, traveling across timezones, or staying up very late for some reason. One of these things may happen every 2 months or so.

I feel mentally good, I'm in shape and in general good health according to yearly checkups. I've been drinking coffee for about 18 years now.

You have good habits. I think six hours may be leaving you with sleep deprivation, though. Despite a lot of pop science about different people needing different amounts of sleep, it seems pretty likely that the overnight chemistry just takes a certain amount of time.