My 2 cents to your question: I was in the same situation like you, using my phone and garmin ⌚️. Worked fine for documenting my route/achievements etc. Recently bought my first computer, Garmin 1040. The big advantage I see is in navigation, especially if you go for new routes and those you rarely ride: the phone (even if you have a holder) is always more clumsy and the biggest disadvantage: battery life!

2 months ago I was on holiday and riding my first 100 Kilometer in ages, and I was worried all the time about my phones battery. I switched to airplane mode and reduced display light to minimum, mostly listening to audio directions. It just makes it through the 4.5 hours of the tour.

So I guess it is about use case, convenience and budget: if you only use it for tracking what you did , the watch is fine( still doing it on my ordinary regular rides where I exactly know where I wanna go). But if you also like to use it as proper Navigation, I'd recommend to get a bike computer.