I think you are having this backwards. Clerics have the highest survivability out of all classes and rule any kind of content in this game. Jokingly, people call this game “Tree of Clerics”, requested a cleric-free PvP and other stuff. Clerics are incredibly strong and deal huge damage, even if they go full CON (the defense stat).
On the other hand, if you intend to play Chrono3, you have to pick between being a masochist that does no damage or being a masochist that enjoys getting their balls kicked (okay, let me explain…).
As a Cryomancer, at least from my experience, your damage wont be the highest. You might be tanky, but your damage will be…not that good. It was painful for me to rob through R7 content, not exactly fun. But Cryomancer has a good amount of CC and are generally fun.
Pyromancer, equipped with an Agny Necklace (it boosts Fire Spells by a lot) will melt everything that touches their fireballs. However, as fireballs are considered enemies, you can shove them around with melee attacks. This makes swordsmen and autoattack Clerics kick your balls around, leading to you dealing absolutely no damage.
This is where Linker however comes into play. With Linker, you can link your fireballs to monsters and use Hangman’s Knot to tightly wrap them up like some kind of present (if you are fast enough to use HK, before people kick that fireball of yours away…) and deal massive damage.
Both are wanted endgame and in a well coordinated party, Pyros will definitely shine, but so do Cryos. It’s up to you what you prefer to play, honestly.
And sorry for the wall of text.