If you really want a solid tank, then you should level up Kashhrek. He’s a solid tank.
However, I agree with @Infinite that given you are newish to the game (and therefore have relatively few heroes and limited roster space) you should focus on heroes that give you a lot of utility. Kashhrek really is a tank, and not a hero well-suited to titans or raids.
Healers and snipers (like Sabina and Sonya, for example) are useful in raids, wars, tourneys, challenges, and defense, so you get more ROI on developing them first.
You mention that you are reluctant to have Elena as your tank. You’re right that she’s not the best tank. However, consider why you want your defense to be super strong. If you are really trying hard to be in the top tiers of trophies, then focusing on defense may make sense, but there is a theory that your defense doesn’t have to be stellar, and even if it is, it is always beatable.
You should also know that some smart people have put a lot of though into where a given hero is ideally placed in your raid defense. You may find that useful.
Now, while it is tempting to focus on your five-star heroes, many will advise that you focus on three- and four-star heroes first, until you get a number of maxed heroes. Exactly how many is up for debate, but you currently have Bane, Graymane, Berden, Azar, and Hawkmoon leveled among 3* – not too bad a line up (Greaymane is not my favorite, but I assume you landed him before Valen).
I recommend that you pick one hero of each element, and level those five heroes (using only feeders of the same color). Start with three- and four-star heroes, and defer working on Elena and Zocc. Five-star heroes consume a lot of resources, and it will take a long time to max them. There are rare ascension materials needed to get to the fourth ascension for them, which makes the process even longer.
My two cents:
purple: Sabina (you need a 4* healer) Baltazar (you have his costume)
yellow: Gan Ju since you have him at 3.40
blue: Sonya untile she’s at 3.60, then Kiril (because he is great) or Valen (he’s fast) or Ulmer (since you’ve already invested in him
green: Brienne
red: Gormek
To answer your original question: Sabina. I actually think Sonya is a better hero (I still use Sonya today in wars, challenges, and tourneys and I am level 68). But, you only other maxed purple is Layla.
Note that Sabina and Sonya both debuff enemies, and that overlap is not ideal. It would be great if you had, instead, Rigard who cleanses. But, you can only work with the heroes you have, right?
You didn’t ask, but I would go with this lineup for raid defense: Bane - Sabina - Kashhrek - Elena - Sonya
Good luck!