I would max tiburtus as a primary titan hero. Higher survivability than 3/70 isarnia. But that’s just me. Cant go wrong leveling either first but i agree proteus and tiburtus both need leveled for sure
I generally have found maxed 4* are better than 3-70 5*, so yes, max the 4* first. Some are good enough to keep using even when you are maxing 5*, like Proteus and Tibs, Sonya for the dispell, Boldtusk and Grimm when you get them.
Boril and Cyprian are useful early-mid-game heroes with the riposte but later on there are too many ways to counter them (like Sonya’s dispell). I still use them for last flags in wars sometimes or 4* tournaments but that’s about it.
3* are useful for wars and 3* tournaments. Don’t neglect them too much.