I’d be focusing on 4* still for where you are in the game. You’ll get way more mileage out of them, and way sooner too.
Red: finish Wilbur and Falcon, then Scarlett. Wilbur, Boldtusk, Falcon, Scarlett and Wu make a great Nature Titan team.
Yellow: finish Chao and Li Xiu, then I’d personally take both Hu Tao and Danzaburro to 3-60 and decide which you want to max first, or if you have another yellow to work on by then. A lot of people would say otherwise, but I don’t regret maxing my Hu Tao, and I’ve gotten a lot of use out of him.
Green: finish Buddy (I’d finish Brienne too, she can do amazing things), then Caedmon, then Melendor
Blue: finish Kiril, then Triton
Purple: finish Proteus and Sabina and Tiburtus, then Rigard