I wouldn't put Kroak in the role of summoning caddy tbh. He has way higher damage potential with just an Umbral Spellportal or teleporting him into enemy lines. Regular Slann don't do any real damage, so it's better as your summoning machine (plus gives you access to double teleport, which is a very nice tool). Running the math on summoning, even with max roll on Astrolith Kroak can only just get the Dread Saurian out Turn 2 assuming no dispels/unbinds of Balewind or Cogs. You won't hit 24 summoning points till Turn 2 at least, so your summoning options are still in the same bracket as a regular Slann.
Keep in mind that you only get one Endless spell per Wizard per hero phase, and you may need to be recasting Palisade, Balewind and/or Cogs if they get unbound or dispelled by enemy Wizards. I would also cast Palisade first every time, as unbinding and dispel both require Line of Sight (which means until the Palisade goes away, you can keep your Balewind and Cogs safe behind it).
Predatory Spells can also be a serious risk, because the player going second always gets first move. Jaws, Swords and Gravetide are all bad anyway, don't waste points and casting slots on them. Pendulum and Gemnids are the way to go if you want consistent damage (Gemnids is particularly nice as they always trigger and their debuffs wreck both melee and shooting units). Another bit of utility I'd consider, mainly because it's so cheap, is Soulsnare Shackles. If you land it in the right spot, you can severely disrupt enemy movement (not simply if it triggers, but just because they will want to avoid it). It's especially crippling for melee hordes, as it's worded as 'per unit' and affects in an AOE, rolling separately for each. Halving movement or forcing them to go around will buy you extra shooting time, or another chance to summon free units.