Wow, and thank you!

So yes, I'd definitely say the external mic is a must have. Unfortunately they've made it so that if you want to use an external mic you have to buy the Media Mod, and that itself costs an extra $79US when I bought mine.
But it's worth noting that I've heard the Media Mod for the Hero 9 is much better than the Media Mod I have.

Also now I believe the Hero 10 is out.

OH, and! There's a Chinese based company called Insta-360 with competing action-cameras to Go-Pro, and I think they might be offering some better features than at least the Hero8/9. But I haven't tested them myself so I can't say for sure.
But as far as video goes on my Hero 8 I do like it.

Depending on what matters to you though a lot of Youtubers use DSLR cameras from Nikon/Canon/Sony etc, and they of course also have external microphone capabilities.
Some people will record video/sound on the DSLR, and also with an external microphone to the computer. They then download the video to the computer, and sync up the microphone's audio with the camera audio based on the audio the camera also captured. They will usually clap 3 times when they start, and that is easily visible in software for syncing.

Those make sense for recording in a studio/at home, but they aren't good for anything where you are walking around whereas the Go-Pro is small and can be handled with the "selfie-stick" while walking.
The Go-Pro gives you the ability to record in 4K, but not everyone wants to do full 4K because it's more intensive to work with in an editor on the computer. Also means longer render/export times.

I like my GoPro, but I'm also hesitant to recommend it because it's a lot of money and I don't know for sure that it was worth it for what I do.

Hope that helps.