@Prodromakos, the first thing I’d suggest is that you feed one hero of each color, as you get the 20% bonus mentioned. In other words, work on one rainbow team. I generally suggest this until you have ~30 maxed 4* or 5* heroes, before focusing on fewer heroes.
Your best rainbow set looks like Danzaboro or Li Xiu (yellow), Buddy (green), either Grimm or Kiril (blue) (Both are great heroes!)
Having a 4* healer makes many things easier, so I’d lean toward Kiril before Grimm, but either choice works.
Work on 3* heros when you don’t have 4* heroes to work in a given color.
If you can avoid feeding 2* heroes, I’d strongly suggest stopping now. Later in the game, 3* heroes are of limited value, but 2* heroes are of no value.
FYI, when you are done with Buddy if you need a 3* hero in green to work on, Brienne’s attack buf when used well is more powerful than it might seem. Especially if you pair her with Gunnar’s spirit link which multiplies the number of hits your heroes take, rapidly increasing her damage buf.