Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

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If those are your only options, get the Amazfit. The FE is pretty much a rebranded Galaxy Watch 4, which has quite crappy GPS (I should know, it was my former watch). Fitbit charge 6 doesn't have the best GPS accuracy either.
I can't comment on Garmin watches, but does he already have an iWatch or a Samsung Galaxy watch? I track all my rides with my Galaxy watch with the strava app on it. Just food for thought. When I was looking at Garmin I was lookin at the Fenix models.
I am using Samsung Galaxy Watch that syncs with my Samsung Galaxy phone
Pixel Watch 4 lasts 2 to 3 days, OnePlus lasts 1 week using their default faces, Galaxy Watch also like 3 days
Neither does galaxy watch. These watches last lole 1 day at max meanwhile Garmin can last for a whole week in a single charge.
For most a garmin a more than enough and that battery is a huge win. I went from galaxy watch to fenix and would not go back. Sounds like we are aligned :-)
Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

Garmin
Forerunner 955 Series
Best value, clear MIP screen; limited smartwatch features.

Garmin
Fenix 7 Series
Durable, multi-week battery, flashlight; high price, buggy software.
Garmin
Instinct 2 Series
Rugged, solar multi-week battery; bulky, small screen readability issues.

Garmin
Enduro 3
Multi-week solar battery, lightweight; but bulky form factor.

Ranked #1
Oura - Oura Ring Series

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Apple - Ultra Series

Ranked #1
Garmin - Enduro 3

Ranked #1
Garmin - Fenix 7 Series

Ranked #1
Garmin - Forerunner 255 Series

Ranked #1
Oura - Oura Ring Series