I'm interested if there is a rule in any of the qiraats which says something like: "when the fatha on any letter precedes the full mouth sakin letter (as dad, sad, qaf etc), it must be pronounced as 'a' (like 'u' in bus) instead of being pronounced as 'e' (as 'a' in cat).
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The letters of tafkheem (referred to some as "full mouth") do not affect the vowel of the letter preceding it. The vowels are always pronounced with tafkheem or tarqeeq correlating to the letter the vowel is on, not the preceding letter. So, for example the fathah on the letters
in the word
are pronounced with tafkheem (heavy and with the sound focused up to the roof of the mouth) but the fathah on the
in the same word is pronounced with tarqeeq (with no heaviness and the sound is not focused to the roof of the mouth).
Also, we highly recommend not comparing the correct Qur'anic sounds to various sounds in English, as the comparison is only very approximate and does not relay the true correct sound. In addition to this, there are various dialects and accents in English throughout the English speaking world, and one area or dialect may pronounce the "a" in cat one way, and another area or dialect may pronounce the sound of the "a" in "cat" in a very different way, and it could be one is very far from the correct sound of a fathah in Qur'anic recitation and the other only somewhat similar.
You are welcome.