They should've either said "F" every time, or said "fuck" every time, and accepted an R rating

Alternating each time feels weird. In fact, saying "F" at all feels weird, but I understand they wanted a PG-13 rating.

I think they should've made a meta joke about it. Like maybe Khalil could've said "By the way, did you know there are limits on how many times you can use the F word in a PG-13 movie? As a result, they often have to use weird substitutes, like "F"." Then Starr could've said "What does that have to do with anything?" Then Khalil could've said "I have no idea, I just somehow felt like mentioning it."

It would feel very out of place, but so does saying "F". At least that would be a good way to acknowledge and apologise for the awkwardness, while also making fun of the MPAA for their ridiculous rules, and explaining the situation to people who aren't familiar with the rules.