I want to add/connect two more TVs (using two more Ultra devices).
Will I also have to re-sign up for already purchased channels and be charged for the additions?
Your subscriptions should be good for your ACCOUNT, not your individual devices. As long as you link the new Rokus to the same Roku account you should be good to go.
Note that you may need to authenticate each of the new devices with each of the subscribed channels so the channel knows your device belongs to your account with them.