I'm glad to see a reference to Converge from BFZ, but this is a really weak uncommon. (Compare the common Nessian Asp for e.g.)

Solution: It should've been a 0/1 that gets two counters for each color. That way it's still the same in a 2 color deck but gives more reward for splashing extra colors

its creature types are more relevant than the asps and being able to be a 7/8 for 5 mana is arguably better than being able to be an 8/9 for 5+7 mana

The decision to make it common or uncommon balance-wise is probably mostly dependent on what kind of metagame you want your limited environment to have. The number of common slots you spend on large creatures is going to greatly affect the meta, and so having a large creature like this be a common or uncommon is dependent both on what you want the meta to be like, and on how many other common slots are already occupied by similarly sized creatures.