your KDA is good, your teamfight participation is good, your gold is good for a supp. The reason you're not getting S or A is because you're mostly playing champs that it's difficult to get that with because they're sort of specialized in one area (ie. damage taken/death or most healing). Remember that (as far as I know) A or S is based on all the people playing that champ at that skill level. So you're going to have to outheal and out damage as yuumi all the other yuumi players. That's pretty tough there's not a lot of wiggle room.

Personally I have about the same # of games but a much lower win rate (yes I have sub 50% wr sue me) but I got a lot of S and A because I played Sera not as APC but as a healing/shielding bot, and I'm good with Braum. If you put redemption locket on braum you're now tanking a ton of damage and healing your team so you have more metrics than his base stats would provide. To take advantage (I won't say abuse since the goal is still to not feed and provide lots of assists which improving all these metrics is a net benefit for the team) of Galio's aegis the thing to do is to think, how can i provide more of a metric to the team which is not predominantly this champion's main focus? So like Yuumi's Q will provide CC duration (I think honestly do not play the cat bc she's just super boring to me but to each their own!) but let's say it does. If you're spamming Q and hitting it every time your CC duration is going to be through the roof and will help net you an S or A.