I’m a happy user of an Apple Watch 6, with cellular, very handy when running to not have to carry a phone in case of emergencies. Does everything I want, routes, mapping, intervals (with this app www.workoutdoors.net). Also does swimming etc.
Add in this app for a bit of info on RHR etc Training Today (trainingtodayapp.com) and this one for sleep tracking AutoSleep (tantsissa.com)
Can’t link up to any power meters that I know of, but like you I have a head unit for that. Also significantly cheaper than the Fenix!
My summary is if you want are an iPhone user and want a great smartwatch that is good enough for most sports, the Apple Watch is great! You’ll need to be prepared to charge it daily. I wear mine overnight then charge as soon as I get up.
If you want a primary fitness watch to link to ant+ devices and for longer 12 hour plus events that is an ok smartwatch then look at the various Garmin devices. 945 is a “lighter” less robust Fenix 6 IMHO. Fenix 5 plus is a cheaper option that still has mapping.