customs and cats

I expect that means 'within 30 days'

You should be good

No, with a rabies vaccine, it takes 30 days to become effective. They mean 30 days since the shot - if it is a first rabies vaccine. If this was an annual booster and you can show previous vaccination and no lapse in immunity, then you should be okay. Since most rabies vaccines are good for one year, or even three years for dogs, then requiring it within 30 days wouldn't make sense!

The key is getting the international health certificate signed/endorsed by both a vet and the Canadian equivalent of the USDA - the CFIA, I think.

When we brought my cat we had to get the health certificate (not all vets can do it and it was I think over $100)  when we got to Punta Cana airport - they didn't even acknowledge I had the cat let alone ask to see any papers. 

We brought our dog into the DR in December 2020. We had to present copies of the health certificate and pay 1000 DOP processing fee.

I think they are more concerned about dogs than cats

All animals are regulated.

Remember...nothing is applied consistently here. Rules, regs will change over time and be applied differently for different people and situations.