Rucking??

rob.goodwin said:

What activity would be accurate for tracking? Walk? Tactical? Hiking?

They will be all equally accurate. The type of the activity you select, has no influence on the accuracy, whether we speak about the GPS, the HR, the Calories, or whatever else. There may be slight variations in the data screens available during the activity, or in the statistical data (for example at Running you can have also Running Dynamics, and generally more data and graphs), but I do not remember any significant differences between Walk and Hike (I think I never used Tactical, so cannot speak for that one). 

Personally, I'd tell rucking is closest to hiking, but if you care about badges and about the user level, you may prefer using the Walk activity, because you get more rewards and more points for walks, than you get for hikes.

Just use walk or whatever.  This is what I do during work- I'm typically carrying 20-25kg of equipment on my back.  What happens is your heart rate increases and that's it. Your watts or calorie expenditure is based on that and over decades of study now, it can be fairly accurate.  It will never be more than ballpark and you should be using other indices, but that's good enough for most people.

You can even copy existing activity (walk/hiking) and rename it to rucking. This way you can have custom settings for Rucking.

I use walking for activities on flat terrain and hiking for every terrain with bigger elevation differences - I just want to see different data on my watch and elevation is deciding factor :)