Leveling 3* is quick and gives you a set of heroes to fight with. I would finish Bane, Belith, Balthazar and Gunnar first.
Please consider saving your talent emblems for 4* and 5*, you’ll need them much more there than your 2*s.
If your goal is maximum speed on growing your bench, then leveling 1 hero per color with same color feeders is a good way, because the feeders give more xp. If you would like to get a specific hero up quickly, then you could feed all feeders. I have been doing same color feeding and advise that for my alliance family mates also.
Heroes worth leveling per color imo
Blue: Kiril, wait with Isarnia because you probably don’t have the ascension materials for her and check the forum for experiences on Captain of D. Boril is ok on mid-level, as a tank for example, but you probably wouldn’t use him after diamond arena.
Red: Boldtusk is very good, Colen is ok
Yellow: Wu is a must for titans
Green: Melendor, then Kashrek - Kash can also serve well as tank for a long time.
Purple: finish Cyp, Rigard
Bt, Kiril and Rigard are all excellent healers,you are lucky!
Some links to start with: