I’ve been back on my Apple Watch Series 2 and to be honest I haven’t missed the series 6. The fitness tracking across the two devices is on par (for me) and the new additional of a blood oxygen sensor whilst I guess useful having a history of data isn’t actually doing anything proactive yet. I found that it was failing about 50% of the time, until I found you need the watch higher up your arm to get a good reading.
Until Apple does something proactive with this sensor or improves it’s reliability I’d stick with your current watch. If you want to track your blood oxygen then get yourself a finger sensor for around £20.