What number of speeds is this?

If it's 9/10-speed, you can use a 9-speed MTB derailer and run as big of a cassette as you please.

If it's 11-speed a GS derailer will handle a 32t. A Tiagra RD-4700-GS is even rated to handle a 34t. With either one you may be able to get a couple more teeth out of it depending on your bike.