it’s really sad that after build the T5 destroyers but can’t use them to complete the mission. 
Sigh, you don’t have to play with the R15 ship, you just need to place it in your combat slot and you can play your R14 destroyer freely.
Actually to me what is sad is that you have to grind to upgrade Tharga using different ships. Totally different method and can’t enjoy using it while trying to upgrade it. Unless ofc you don’t have higher tier ships. Would love to be able to USE Tharga while trying to upgrade it but i’m sure they have there reasons so I will just go with it.