Ravenous Bugblatter Beast of Traal
Let me flip this around. Do you want to spread your attacks and mark as many opponents as you can, or do you want to focus fire and drop them as quick as possible. These are mutually exclusive.Wow, thanks for the input. So going for damage is potentially suicidal as a cavalier due to all the marking one may do.
I looked at the eventual charge for advantage on future attacks and figured gaining advantage would be great for GWM.
Is going 2-handed weapons completely foolish, or is it more of an issue of not allowing yourself to do your job as well as you could?
If the former, then you won't be getting the same millage out of damaging feats and two handed weapons because you aren't going for action-denial-through-death. Not that these feats are bad, but they are all weaker and it then becomes if others that haven't been weakened give you more utility.
If you're going for the latter - well, there might be better subclasses for damage potential where you can focus on their features.
So, your Unwavering Mark gives them a good reason to attack you (if they are within 5'), so good defenses are a positive - unlike other tanks, you don't risk making yourself too uninviting a target that it's worth the OA to just bypass you. Here is another reason while shield gives benefits over two handed weapons. Feats and features to improve your defenses / reduce damage are useful.
Sentinel is also useful, locking them down to keep one within 5' if it tries to leave. A reach weapon is actually a detriment to this. This is another shot against PAM. (Though the rules-legal (but many consider it cheesy) combo of a staff one-handed, shield, and PAM would avoid it - giving you a better defenses, only a 5' reach, and a reliable bonus action attack since your mark bonus action is at most STR times per day.) This is completely influenced by campaigns that only last until 10-12th and wanting to get it early enough to be useful. If you are planning to go higher then Hold the Line fits in well, if not exactly the same.
With CON useful for HPs and protecting your mount only, I wouldn't delay all feats for CON 20. Some will give you more utility earlier than that.