Thankyou for your feedback!
Yes, the CR is the thing concerns me the most. I decided to go up to CR 8 because the polymorph spell allows an 8th level character to transform into a CR 8 tyrannosaurus (the beast with the highest CR in the PHB), so I assumed that a level 20 character could somehow summon a creature of equal power, not to mention that I consider the ability to summon a tyrannosaurus a mandatory target for a class called "Kaiju Tamer". Also, a higher CR would also add more room to apply the effects of the "Release theKrakenCryptid" feature.
But yes, as it is now, this subclass is way too strong when compared to a Moon Druid (not to mention when compared to a Beastmaster Ranger). However I want to try other solutions before (probably inevitably) reducing the CR (E.G. removing or halving the proficiency bonus to AC, working on a different action economy, adding a spell slots cost to special actions...).
This is an interesting solution, but it would almost be just flavor.
I will definitely consider this, even if I find this flavor very thematic, since I consider mythological monsters such as krakens, scyllas, charybdis, hydras, aspidochelones or leviathans as proto-kaiju. XD
However, it would be interesting to create different themes, finding for each a pro and a con, such as the amphibious trait and the low walking speed of the maritime one.
I'll do my best!