Which fitness tracker/watch for my Polar H10 and specific requirements?

Hello everyone,

I would like to start sport again after a long break. It's important for me to customise my training and get an overview of the relevant measurements.

Garmin has an extensive range of watches and trackers, but as a layman I'm a bit lost and am turning to you.

The following points should be fulfilled when choosing a suitable watch:
- I have the Polar H10 as a chest strap, which should then display the heartrate (HR) on the watch during training
--> I have at least heard that the HR then replaces the watch's own value, but that all other values on the screen display remain unaffected. Is that correct?
- I would like to be able to call up and start training plans created in Garmin Connect on the watch (e.g. interval training)
- Display typical sports data on the watch during training (speed, pace, time, distance)
- for running, cycling, hiking
- I would like a slim watch for my narrow wrist
- Price up to 80€, second-hand watch

I have my eye on the Vivoactive 4S and the Forerunner 45S. The Vivoactive can do a lot more than I actually need, but can't display pace as far as I know. And the Forerunner is probably not supposed to fully integrate the HR values from the chest strap into the screen display? Surely you know better.

I look forward to tips and suggestions :)

Greetings