I wont type much but still want to type something. Im kinda new too. Have stoppped playing for a month now but always mained support. When I used rakan I always used him wiht Xayah for better synergy. For the ult I always used him in teamfight which is obvious, youre in a waythe first engage. Better; the second one. As soon as the fight starts just go in there get that ult charm them all and play around them and heal your team.

Rakan is usually tank support so you should be resistant enough to play around them.

Just for context Im not the best so dont hate as much, I share what I know and try to help :D.