Embleming up a hero is a long term investment, I’ve reset more than a few 4* just to emblem up my 5* faster, and I really regret it. Mostly because turns out in the end I still use those 4* quite a bit, and restored them back to +18 later down the road anyway, and I lost tens of millions of food/iron and probably a literal dozen reset emblems in due process.
Poseidon+12 vs +20 isn’t a whole lot of difference, but put +18 on BT and he basically becomes a 5* especially since you also have his costume. Keep the emblems on him.
Also btw, BT with costume only needs a lv 5 mana troop to fire in 9 tiles, Anzogh and Red Hood will still need a lv 23 troop to do that. So from that perspective BT is a much more valuable healer than either. Sumle being slow and no costume bonus, even with the 4% mana bonus the best you can do is have one tile with a lv 5 troop, so I wouldn’t even bother putting a mana troop on the guy since it’s basically wasted.