The “full armor” equipped Type 10 is actually 48 tons (this is somehow missed by everyone). So it’s only 2 tons or 4% lighter than the previous Type 90. The upgraded armor material is claimed to be 12% lighter with the same protection. Doing the math (including reduction in the volume), the amount of protection increased for all angles.
K2 could have been much better by the 44 calibre main gun instead and making it more focused on fighting in Korean Peninsula (perhaps a mortar like Merkava for fighting against a human wave attack by North Korea). But instead, South Korean government had to add bells and whistles that would only help on exporting.