Here is the answer to your question:
You are not required to choose to reroll the 6 before performing a second Feat.
Here is the exact rule from the Feats section:
You don’t have to execute multiple Feats reroll all at once; you may always look at the result of a single rerolled die before choosing to perform another Feat.
Thanks for the kind words about our small game! We really appreciate it and I'm glad you're having a good time.