Explosion and Self Destruct may have power, but they also knock you out too. You generally want to maintain your advantage over your opponent by knocking their Pokemon out while keeping your Pokemon in the game. While Explosion/Self Destruct will probably either knock out an opponent or do a hefty chunk of damage, it also means that in the best case scenario, you haven't actually gained an advantage over your opponent at all since you both lost a Pokemon, while in the worst case, you're actually making it harder for yourself to win, since they might tank the hit, but your Pokemon will be KOed no matter what. Explosion is gimmicky at best, and if it's power you want, there are better moves you can use to get that.
As for if a move that had a base power of 250, but you had to recharge for two turns afterwards would be broken, I'm gonna say no, because that's two turns you're unable to attack, two free hits that an opponent can chip away at your team, and two turns you could've spent potentially knocking out more Pokemon and doing more damage than you could've with that one move.
Hope this helped!