Yup, jack and serendi are very very good. If you have the ancient coins to spare after getting the daily Mercedes(rock attack) genes, Mu (paper balance) is the best of the ancient coins heroes. The collab heroes are also worth picking up. Evan (scissor AoE) should be the easiest as he can be farmed easier than the other 2. As far as the event shop goes. If you have not already started hoarding them, get the daily mod pots as you will need lots of them later for modding your gear, even the 4* and 5* ones as they are especially difficult to get without spending hoards of gold. If you already have a dislike for it thats perfectly normal, but i suggest doing dimensional breakthrough every day for gold and Lena genes (scissor balance). Lena is a great hero with tons of utility, charm chance, 2 turn 100% immobilize at skill lvl 1 and %hp damage and weakening. You'll hate DB very quickly but the gold and Lena are worth it, even just autoing a few stages till you're wiped out is helpful.
Like ghengis said, if Krut ever comes back you will want to get him, he really really good in that hes an attack type which are normally lower defense, but krut has defense type levels of HP and defense while still having very high attack.