spending rank progression points

What's the most effective way to spend them? Up until now I've put all of mine into Guard Killer and Defense Against Guards, but the first two levels of Arrow Cart Mastery look really strong now.
I'm also a bit concerned about the wording of level 6 Guard Killer - "All guard killer bonuses now also affect enemy lords and supervisors" - so they didn't already? What counts as a guard if supervisors don't?
Lastly, does anyone know if there's a rank cap, or can we theoretically keep gaining enough points to max everything out eventually?

1. Arrow cart mastery is essential now.

2. Non guild claimers are guards. Usually labeled scout etc.

3. I would assume no cap seeing as some people are bryond 90-100 already but no person has gotten high enough (to my knowledge) to max everything yet.

Arrow carts when upgraded will hit for 2.5k + a tick it it's by far the first thing you should upgrade

im thinking arrow carts will not remain as strong, but I would still get the first "rank" of ac mastery that gives you extended range. (edit: they will definitely be changed)

some people liked to save up and go for the increased supply capacity but I personally am just levelling up the ranks in terms of ascending requirements (i.e. I got all the abilities that took 1 point, then 2 points, then 3 points, then 5 etc.)