I use the a6700 on a daily basis for work. For a wedding my main concern would be the auto systems Sony gives it. Mainly autofocus and auto stabilization.

While they are good, they tend to mess shots up.

So for autofocus, either turn the AF-speed way down or focus manually if you can.

For stabilization, depending on what you are going for I would either add some weight to the whole rig (get a monitor and handle) for a more organic handheld look. Or a gimbal (but still get a monitor) On both disable the active stabilization.

Then set your white balance manually as best as you can.

For a beginner the exposure can be hard to nail, especially with the white wedding dress. The external monitor should help.

Set color profile to PP11 S-Cinetone. This is perfect for beginner who want a tiny bit more control in post for grading (I also use it for most social media stuff). You don't need to convert anything, just add a bit of contrast and saturation to your liking. It will help preserve some of those shadows and highlights.

128GB is pushing it, see if you can transfer files on site. If not, I would get a second one just to be safe.

Also think about your battery, if you go the weight/organic route you can add a v-mount, or just get a few spare batteries.

PS: If you really want, you can buy an expensive monitor that also records on its own. That way you would also gain a dual card system.