First, fuŝ- is a (verbal) radix, stem word. The usage above is perfectly fine, where English would use two separate words.

• kantaĉi = sing ugly (la rano/knabineto kantaĉis = the frog/little girl sang dreadful)
• fuŝkanti = sing with wrong technique/mastery (like drunk)
• fuŝkantaĉi = both

Here it is a difference in nuance.

Your aim would be something like:

• malbelkantaĉi = ***

Which is an ugly word creation itself.

To strengthen aĉ on may use:

• kantaĉegi = sing very dreadfully
• tre aĉe kanti

By the way kantaĉi one would not translate natively with sing+adverb, but more something like whine, belt out, yodel (?). But I am not native.

First, fuŝ- is a (verbal) radix, stem word. The usage above is perfectly fine, where English would use two separate words.

• kantaĉi = sing ugly (la rano/knabineto kantaĉis = the frog/little girl sang dreadful)
• fuŝkanti = sing with wrong technique/mastery (like drunk)
• fuŝkantaĉi = both

Here it is a difference in nuance.

Your aim would be something like:

• malbelkantaĉi = ***

Which is an ugly word creation itself.

To strengthen aĉ one may use:

• kantaĉegi = sing very dreadfully
• tre aĉe kanti

By the way kantaĉi one would not translate natively with sing+adverb, but more something like whine, belt out, yodel (?). But I am not native.