For navigation you really, really want a bike computer. Even following a breadcrumb trail would is leagues better than anything on a watch.
Independently of that, I reckon you will find smartwatches quite useful. Rather, you’ll likely find a use for it. But I wouldn’t want to use them for navigation on the bike.
A head unit is generally easier for nav, but I have used WorkOutdoors on my Apple Watch well enough (e.g. an 80k ride through the mountains on a rental bike in Kos on holiday). Also RideWithGPS have good nav support for Apple Watch with navigation notifications via haptics and sound (https://support.ridewithgps.com/hc/en-us/articles/4419025011611-Apple-Watch). There is nothing native on Apple Watch for GPX routes though so you will need a third party app like these.
I’m a happy Gentler Streak user. I guess it may not suit the super lot here, but I’m a dad with a 4 year old and a busy job. It does well with looking at everything not just my TR rides.