Help adding relay for thermostats
I would try connecting the C-wire to the existing 24V COM terminal on the relay and seeing if it works
Your relay, thankfully, makes its C (Common) connection accessible (not all HVAC relay modules of that nature do).  So, given that your Taco's transformer secondary is rated at 15VA (as per the label on the transformer) and the Nest only pulls about 5VA of power, based on this document, the Nest shouldn't have any issue working with your relay once that C wire's hooked up.