Rucking

Just use the Hiking activity profile for that. Is there anything you miss on the screens of that Activity? If so, you can customize it, and add the missing data fields.There are thousands of sports that are not listed on the devices or in Garmin Connect, and they also mostly do not need to be, since you can track pretty much every sport with one of the available profiles.

Yep I'd love a field for "weight" so I could put in how much weight I carried on the ruck

Yes, and then the app can get close to calculating calorie burn by using walking pace at different body weight I think. 

Well, the activities have a note field, so you can record the pack weight there. And since calorie consumption is based on heart rate (which increases with a heavier pack), the calories already automatically take pack weight into account. I'd probably suggest using the hike activity, which doesn't try to estimate vo2max like walk (although running activities override walking vo2max, so if you also run it probably doesn't matter).

Good to know how the calculations work. This makes sense. Thank you. 

I found these 2 datafields that will allow you to enter and save ruck weight to the activity record.
Rucking calories: https://apps.garmin.com/en-US/apps/3bf3d9aa-8493-4ace-91c8-d63e1f4cf7ca
MET: https://apps.garmin.com/en-US/apps/d487afae-e747-4cbc-ab3b-87f447a21234