Unsettled funds/Reg T question...

I could just as easily ask TK support directly but like to post questions here so everyone can benefit.

First an example...

I have a cash account and I buy XYZ and also buy ABC on the same day with free and clear funds. I then sell XYZ the same day and have unsettled funds in my account. On the next day I sell ABC which adds to my unsettled funds total. On the same day as the ABC sale I buy DEF understanding that I have to hold until the funds with which it was purchased need to settle. (T+3)

Could I safely sell the DEF stock on the T+3 date of the XYZ sale or would I need to wait until the T+3 date of the ABC sale? In essence is the stock purchase strictly tied to the preceding sale or is it tied to available buying power and/or unsettled funds?

Thanks for your help!