but there was such a missed opportunity to shake things up.

They shake things up by improving significantly upon the establish but stale concept.

it's odd that TemTem would stick so rigidly to it

It doesn't really. The MMO-aspect, overworld and music are the most evident examples, but even within the more similar battle system it's clear as day they're doing something different. 2v2 focus, Synergy, Hold, Stamina, etc. And the breeding system is completely different other than the concept of putting a male and female together.

A meta-difference is the fact that Crema is regularly updating the game and balancing the competitive meta. It tries to make every Temtem competitively viable. Sometimes, it even goes as far as changing the type chart.