What watch for Kilimanjaro?

I’m going up Kilimanjaro later in the year. I’ve had an Apple Watch for a few years and it’s fine, but for Kili I want an oxygen sensor. My series 4 doesn’t have one, and the recent legal difficulties Apple have had make me wary of simply getting a newer one. So after a bit of research I’ve decided to go with a Garmin. 

Which one though? Venu 3 or Fenix 7? Or something else? I don’t want a big watch - 42mm is about right - and I don’t want a chunky one. I want HR and O2 sensing. GPS is nice. And if it connects to my iphone for messages and notifications that’d be good. 

Thanks in advance for any advice.