Filing US taxes, where's my domicile?

We are US expats living in Panama. We moved here in May 2019. Our previous residence was in Pennsylvania, but we sold that residence in March of 2019. We set up a digital mailbox in Florida so we could get our mail forwarded to Panama. Using the address from the digital mailbox, we were able to get Florida driver’s licenses. However the voter registration office in Miami Dade county couldn’t process our request for voter registration in Florida because they recognized the digital mailbox address as a business address and not a residence. I am unclear as to what address I should use for filing with the IRS, and a non resident return for 1/3 of the year we lived in Pennsylvania. Thanks in advance.