What are the exact frames of Charizard's grab release anyways?
He gets out on frame 30, like (almost) everyone. The good options are as follows--
Jab. Frame 4 isn't very good for a jab, but it's not terrible, either. The important thing is, it beats any option that isn't another jab or a stupidly-fast move (like Ivysaur's Bullet Seed or Wolf's Down-B). Since some people like to use a tilt or grab, it's a decent option. Also, because it's three hits, it can cancel quickly if they attempt to rolling dodge, and you can still land a hit after an immediate spot-dodge if they do that.
Rock Smash. It is the ultimate spot-dodge punish (damage-wise, at least), so for people that are looking to avoid a hit and retaliate quickly, this does a real number on them. Also, the rock's hitbox is out on Frame 3, so almost all jabs (with the exception of a few characters, like Peach and Squirtle) will break the rock and take heavy damage. If their jab doesn't have much range, they might take the headbutt portion and go flying, too! Also, if they roll towards you, unless they get great distance on their forward roll, the shards could hit them and prevent you from being punished. You can still get grabbed, but many characters don't have the grab range to immediately grab after a grab release, so use that to your advantage.
Grab. It comes out on Frame 6, which is the standard (and also the best, I believe), so it's a decent option when faced with an opponent that likes to shield after a grab-release. Charizard is one of a select few characters that can grab an opponent after a grab-release without having to move, so most people don't see it coming. It would only net you a few extra pummels, but it resets the grab-release, which can be a deadly mindgame when used well. This also helps with diminishing returns, and any extra bit of damage helps. It's easily stopped by a quick jab or tilt, so be careful against more offensive players.
Down-Tilt. It comes out on Frame 8, which is slow compared to your other options, but Charizard leans back during the animation, which can prove to be a solid dodge-and-counter move on anyone who was expecting to have just enough range to punish you. The whole move ends on Frame 27, so even if the opponent dodges the attack, you're only at a disadvantage by a couple of frames at worst, so you won't be punished too badly. This move also has great killing potential especially if they don't see it coming, so that's a thought.
Forward-Smash. It comes out on Frame 22 and the first hitbox stays out until Frame 24, which would normally be considered atrocious as a grab-release option, but when you take into account that it punishes every spot-dodge perfectly (the best ones, like Link/Falco, end on Frame 22, while a majority of the cast end on Frame 25--either way, invulnerability ends on Frame 20 for pretty much everyone) and the fact that it deals obscene damage and knockback, this suddenly becomes much more viable than anyone would've originally assumed. Also, Charizard leans back during the animation, and even more so than with D-Tilt, so if they do a laggy move, you can punish with this. It might cause some whiffed grabs to get punished, and if some character roll behind you, they'll usually get knocked away a bit by the first, less-damaging hit of the Forward-Smash, which helps cover you. Also, one thing I like to do is initial dash backwards and immediately do a Forward-Smash, which creates even greater distance and can be useful, although it is situational.