The build he uses is to survive as long as possible and boost his healing to the max.
I despise builds like that but it isn’t necessarily bad.
He is a team healer lategame this way. His damage will be negible though especially with the SC nerfs. I would have wanted to see a clockwork or Echo on him though.
Your heavy prism on cath is much weirder than Adagios build… Aftershock ok, Clockwork fine… But raw CP doesn’t work for Cath like it works for Lyra Grace Lance Lorelai or Adagio…
Here is how I build cath. (Also poa silver) with and against Vainglorious people and getting upvoted by them.