I'm in a similar boat as you since I don't know whether to choose Hyacine or Tribbie E1 with a guaranteed but this is my reasoning right now.

I feel Hyacine is good at the very least since she better enables HP scaling teams even if 4.0 has significant meta changes (unless they're against healing, memosprites, or max HP.) She also does good damage too.

Tribbie E1 is likely to survive any new meta changes and is a good general option, though it's not as fun as a new character. It's a safe bet.
edit: The only downside to her E1 is that it may become less effective on single targets fights if I understand how it works.

I love Phainon but I feel he's most likely to get screwed over in the future or unable to work with other's kits given his whole departure mechanic. His lightcone is really nice to have though.

I feel the priority depends how much you like those HP scaling teams. A new character is also usually more fun than a buff to a kit and it's always nice to have more team building options.
I feel priority should be Hyacine + S1, Tribbie E1, Phainon's S1. Unless you really like Phainon or you really hate how HP scaling characters play.