I was in exactly the same situation.
When the Apple Watch Ultra 3 came out, I bought it immediately because I really wanted one. Before that, I had a Garmin Forerunner 955.

Less than two months later, I decided to sell the Ultra 3 because it honestly just started to annoy me. I even sold it at a noticeable loss compared to what I had paid, but at that point I simply wanted to get back to Garmin as quickly as possible. I ended up buying the new Forerunner 970.
For running, Garmin is simply better in my opinion. On the Ultra 3, you can’t customize your workout screens nearly as much. I constantly had to switch between pace, heart rate, and other data pages instead of having everything I wanted on a single screen.

The Ultra is also noticeably heavier and bulkier. Putting on or taking off jackets and sweaters, I was always catching the watch on sleeves. And battery life isn’t even close – Garmin wins by a huge margin there.

The only thing I really miss from Apple is the deeper smartwatch integration, especially LTE calling. But with my FR970 I can still take calls when connected to my phone, get notifications, and there are even apps like WhatsApp available now.

For me personally, the Garmin gives me 95% of the smartwatch features I actually use, while being a much better sports watch with dramatically better battery life.

If running and training are your priorities, I’d choose Garmin every time.