Question: I am early in this design but am imagining a fictional world in the age of exploration including classic fantasy tropes of monsters and magic, with renaissance era technology, that is cannons, muskets, clockwork and wacky renaissance war machines perhaps entwined with magic albeit to limited effect or making use of fantastical beasts. I would like the world to stay in this state of technology for as long as possible.

So I indeed ask, What would stop advancements in technology beyond this state? Specifically weaponry?

Answer: 
Interchangeable parts don't.
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No, *seriously* - I don't just mean there are poor tolerances in your manufacturing. I mean that having all those blasted wizards running around tapping into arcane essences of space, time, thought, logic, and reality ... it means that an inch in Chicago isn't always more than seven-eighths in New York. It means a cartridge loaded in your basement might not be reliable in the woods behind your house.

The most practical technology for your scenario is *crude* tech with very *high* tolerances, so that you can't be disappointed. Load up a blunderbuss with thimbles, forks, and hope (and a mass of black powder casually poured from a horn) ... and it has no more uncertainties than it would in our world.
Empire
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The combination of this technology and of magic let someone unite the region. He suppressed war with a heavy hand and discouraged other innovation as causing unemployment.

Central control gave innovation no place to hide, and the emperor did not fear a rival empire would take it if he didn't.