I have full cover phenix

As title, I got my 15th phenix cover. She is 3/5/5 now. I will get my 16th cover by daily supply. Though many players suggest me not to champion her, I will champion her. The main reason is that I am tired of championing 4 star, especially in the new vault system.

I got 29 champion 4 star. 90 % 4 stars are full cover.
As to my 5 star: phenix:3/5/5, OML 3/5/2 LV 375, IM 1/4/5 LV 255, Strange 4/4/2 Lv 255
So, I am think that should I level up my OML, IM or strange to LV405? Or just using My phenix with other champion 4 stars???

PS: I play both PVE and PVP.

The problem with the fivestar tier is that once you go there, you can’t really go back. It might be a better idea to just level up Phoenix and let the other fivestars stay at base level for the moment. Having just one fivestar leveled doesn’t hit you quite as hard as I understand it. You will still need to use Phoenix almost all the time though.
Out of curiousity, what level are your fourstars?

My 4 stars level are around 280. I am a daily player. But I don’t like to grind the node all my life

Unfortunately
that will not change with a maxed fivestar. If anything, the increased scaling means you need to grind more.