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

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my se3 last 24hrs+ NO NEED to use low power mode im very impressed
Okay naman siya for working out. Jowa ko naka-SE3. Charges it siguro once every 1.5 days, take note that she doesn't use it to track any workouts. I'd be wary sa battery life niya when doing long runs.
If you see yourself hating having to charge your watch everyday, Apple Watch is out of the options. Looks like main use-case mo ay para sa physical activities, I say Garmin.
Based on OP's choices, it seems Ultra is out of the budget and a little extreme for their use-case
i do all of that with my Apple Watch SE 3, i cant complain
SE3, not older. So much quality of life improvement, it's not worth going for less. A used Series 10 would also be a safe bet. As would waiting until Sept for the Series 12. It will have a genuinely new chip which will unlock new functionality for the next 3 years to come. There's nothing in the Apple Watch that's bloated. Just turn off notifications (and very selectively turn on for some if needed).
Almost 71yo here. I'm an advanced indoor cyclist. If parent has an iPhone, I would recommend an Apple Watch SE. At $250, it's a bit over your budget, but the added benefits for an elderly person make it worth the extra cost. In addition to heart rate, steps and sleep tracking and activity reminders, Apple Watch offers: • Fall detection, a potential life-saver. • Emergency SOS and Crash Detection • Medical ID - In an emergency, first responders are able to quickly access critical medical information — like allergies or medications — directly from the watch • "Find My" - The answer to "How where did I leave my phone/keys?" I only wish they made Find My for glasses... • Make and answer phone calls, without the phone. I got my first AW soon after I had broken my leg. Made a world of difference. Ironically, about the only time that I don't wear my AW is when I work out because although it's fairly accurate, it doesn't update as quickly as the chest heart rate monitor I use. But, that probably won't be an issue for more moderate users.
I have se3 dont see any pt changing to Garmin especially with workoutdoors. Charging every other day doesn’t bother me one bit
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