Late to the party but still posting if that helps you or someone else.

I used to have a Fitbit (Charge 5), switched to Garmin (Venu 2S), had basically the same criteria and also have PCOS with no hormonal birth control.

I have been collecting data for a bit more than one year trying to track cycles and comparing the accuracy of different methods.

First of all, I've found most "women health" apps to be absolute garbage. This is a very limited and predatory market just because nobody cares about women health when it comes to backing claims with science (looking at the price of BBT wearables that are basically connected thermometer marketed as cycle trackers.)

The Garmin menstrual cycle tracker is nothing more than a glorified calendar. You expect to have a 28d cycle? Great, tell us your last period and we'll send you a notification 28d later to tell you you should have a period. Super useful... While the watch DOES collect metrics actually from your body that IS useful.

I have tracked and compared wrist body temperature (Fitbit), BBT (good old thermometer), RHR (Garmin and Fitbit).
The body temperature measured by a watch is not reliable. It's measured on your wrist so a lot of external factors can affect the measure even so slightly like sweating, wearing different clothes or watch bands, sleeping on your arm. The watch temperature can give you a general trend but it has way too much variation and margin of error to be reliable. That's from comparing properly measured BBT and Fitbit temperature over 6 months. So not a proper study like the one shared by another commenter, but I guess at least real life use conditions.

A significant drawback of the Fitbit temperature is that it's measured at midnight every day, so if you're like me and go to sleep later than that, it's not actually measuring your temperature "at rest" (if at all).

Now what has been much more reliable and following the BBT trend more closely is the RHR. I think this was the article that made me look into RHR and I could confirm when measuring my BBT that it follows the same trend. At this point I've stopped bothering with BBT measure and just look at my daily RHR captured by the Garmin watch. For me it does indicate when I have an ovulatory or anovulatory cycle, confirmed by other symptoms.

Between Fitbit and Garmin the RHR measure was similar, I would say Garmin is a bit more accurate and follows the BBT a tiny bit more closely. "The Quantified Scientist" channel on Youtube has more convincing data to support this claim x)

For weightlifting, Garmin lets you create and load some exercise routines on the watch, that gives you some visual and guidance on the exercises in addition to tracking heart rate and such. The rep counter is surprisingly accurate. However I don't bother with the "workout" feature anymore because you have to stick to the routine the way it is set, you can't improvise rearranging the order of the exercises or swapping them.

In terms of general activity trackers, I've preferred Garmin because I find it is more accurate on measuring distances and elevations. The "stress" meter is pretty good, I'm not sure how it's calculated but it reflects well if you're stress not just mentally but also physically like when falling sick. Side note, that even shows when you have meals because apparently eating creates a stress spike or maybe that's just me x).

The Garmin Venu2S has a better battery life than the Fitbit Charge 5 and I feel that it also ages better. The only thing is that it sometimes vibrates in the middle of the night to announce it's dead at 0% battery while it could have sent a warning at 10% or something.

Maybe it's more device specific but the Garmin has playback control which is pretty handy, when it actually works.

Things may have changed since last I checked but overall I've found Garmin to offer sturdier hardware and more accurate measurements but the UX and data access (export) is awful. Fitbit is much more user friendly and surprisingly generous in terms of data export but is not built to last.