Do you know how to do that printf within a zsh variable? i.e. here:

    function prompt_epoch() {
      p10k segment -f 66 -t ${MYEPOCH} <-- replace MYEPOCH with EPOCHSECONDS but with commas?
    }

EDIT: printf can assign output to a variable using -v. Therefore my function can be rewritten as:

    function prompt_epoch() {
      printf -v COMMA_EPOCH "%'d" ${EPOCHSECONDS}
      p10k segment -f 66 -t ${COMMA_EPOCH}
    }

Nice, thank you!