I've had a Fitbit Alta HR for 5-6 years now, and while I'm surprised it's lasted this long, it is on its way out. It only keeps a charge for a few days and it doesn't seem to stay connected to the phone, sometimes goes several days or weeks without syncing. I am not sure what could possibly be causing that but it's pretty inconvenient when I realize it isn't syncing anymore. So it was time to go.
I'd heard of Amazfit a few times but didn't pay too much attention to their offerings and price and I initially was very skeptical... A device that offers more (on paper, anyway) than the Fitbit for a third of the cost. I have had an Amazfit Band 5 (otherwise known as the Xiaomi Mi Band 5) for several weeks now and it's excellent. I get 10+ days out of the battery, the band is comfortable, the size is perfect, the screen is quite readable even in sunlight, etc. I do question the Zepp software a bit in that it shows it'll do Blood Oxygen, and you can tell that it's doing it at night (red LED on constantly) but the data never shows up anywhere. But for other things like steps, stress, heart rate, etc, it seemed to align very well with my Fitbit.
Another thing I question is the sleep analysis; it doesn't seem to match up with my Fitbit, but when I looked closely a few nights when I wore the Band 5, Fitbit, and a WearOS watch running Sleep as Android, none of them really matched up that great, but the Band 5 in particular never seems to show deep sleep and rarely shows any kind of sleep cycles. I suspect this could be tweaked in software but it's been out for a while so I have no expectation this will happen.
The Amazfit also has a pretty large community of people who make custom faces and third-party applications that can replace Zepp as the phone software. I've tried a few of the third-party apps and I found them clunky and unreliable and when they were installed data wasn't getting reliably stored, no heart rate, no steps, etc, but I plan on trying them again sometime in the future as they did have more features and tweaks.
I think if I ever get another smartwatch or a bigger fitness band, I will start at Amazfit and see what fits my needs.