This is an interesting explanation of the Turkey CU:
https://ukandeu.ac.uk/explainers/eu-...customs-union/
Note how countries that agree a FTA with the EU have little incentive to negotiate one with such as Turkey as they can push their goods through to them via the EU and always buy their Turkish goods from the EU.
Worth noting how that could mean we get a backdoor Trump
I'm not sure if I stated that Turkey can't negotiate FTA's but if I did, that's incorrect. The EUCU is a different animal since the EC negotiate all deals.