I believe you’re going to be told to try to ascend one good hero at a time ,I have made that mistake but a Fully ascended four star hero at Max skill at Level 23 looks good. But if you Fully leveled him or her you will notice a huge difference in power. I will concede further replies to more experienced players. Good luck
Tib might be better over Sabina, because you already have good healers and utility heroes (wilbur, BT, Kashrek) and the team would need a damage dealer more than another healer who is weaker than bt/Kashrek.
5* are often squishy at 3/70, their potential is bigger on final ascension - but that requires rare mats, takes a long time to acquire them. By leveling 4*, you would have a more useful bench while waiting mats for the 5*s.