“Truer” is grammatically correct.

While we cannot speak for other sources such as the Cambridge English Dictionary, we would probably not ever use phrases such as “more perfect” or “most perfect” on our website unless surrounding context put us in an unexpected instance of adapting ourselves for clarity’s sake, in which case we might revise the surrounding context.

When you say “we would hesitate to use the phrases,” are you implying that you wouldn’t rule out using them in exceptional circumstances?

Since perfection cannot be improved upon, we would hesitate to use the phrases “more perfect” or “most perfect” on our site.

I came across the Cambridge English Dictionary definition of the word “acme” as: “the best or most perfect thing that can exist or be achieved.”

Also, Joe Biden in his inaugural speech as U.S. President referred to “a more perfect Union.”

Any comment on “most perfect” and “more perfect”?