What type of olives would you suggest using? Also, any veg friendly substitutes for the anchovies or will I lie losing a key aspect of the spread?

My personal favorite for the green ones are Castelvetrano. Cerignolas are great because they are big, fat and easy to pit. Don't forget your capers! Its not tapenade without capers. For me anchovies are an essential component but it's not the most assertive flavor if you use good ones. So buy good ones and not the "pizza anchovies".

Lucques and Picholines are both great options. You might also try Cerignolas.

Picholines or Lucques.