Using Facing means there is an addition contender for use of the Reaction, meaning OAs will be even less likely to be used than normal
I rarely see reactions needing more contention. Other than OAs,
shield, and Uncanny Dodge, there isn't a lot for reactions IME.
Using Marking as well gives OAs without needing to use the Reaction, easing the constraint imposed by using Facing.
Which defeats the purpose of the first item, creating more contention. So, it is sort of self-defeating.
Using Facing only gives advantage when attacking from the rear, which isn't going to be all the time (but as you say, Rogues will love it)
Yep, that is one reason why I understand
@Charlaquin liking the idea of facing.
Using Marking gives advantage when doing OAs as well, so it's still not all the time, but it's more often that just rear attacks (and gives the tanks some love too).
Hitting in 5E is rarely a problem, so giving OAs advantage
and making them free is overkill IMO. Making them a free reaction would be good enough I would think.
Using Facing means people will be encouraged to think about their positioning & facing, for reasons 1 & 2 you mention above, which encourages a level of strategic playstyle.
Again, I understand why facing is a good option to replace flanking, I just don't see how marking really brings anything to the "replace flanking" table.
Using Marking as well means that OAs are more likely to hit meaning they bump up in the threat assessment, so that movement taking advantage of Facing needs to be more considered.
Again, increasing the chance of hitting is not much of a benefit. IME players are already loathe to provoke OAs, so will disengage if they have to before risking an OA. Only features like those from Sentinel which offer other means to gain an OA will really benefit from marking as the advantage is there and the OA becomes free.
So, using marking is fine if you want to make OAs stronger (not my goal, personally, and I don't think a goal of the OP but I could be wrong), and replacing flanking with facing solves at least one of the OP's issues, but not the second.
Sometimes they'll get advantage through these rules, but not always. Only on OAs or rear attacks.
True, which is much less than flanking does (just facing alone, really).
There'll still be plenty of opportunities for class features that grant advantage in other situations to still be of benefit.
As opposed to just always getting advantage for every attack from flanking just because you showed up to the fight with a mate nearby.
In which case the OP might as well remove flanking and not worry too much about facing, simply declare any attacker in position opposite the target's last attack as "rear", and give that
one creature advantage or whatever "something else" to replace it.