Adding a UD1 would be completely different then plugging a relay board in?
Not sure what the goal is. Do you need more outputs, or just relays?
I'm using a board like this: https://www.amazon.com/dp/B07BLWZQZB...ing=UTF8&psc=1
You can buy 3, and have two spares for less than half the cost of the PMDX-109