Aaron Chaim, what you say makes sense in theory but is absurd in practice. How is it any more convenient to use a gross distortion than it is to pronounce the word correctly?
How is “chumish” any more convenient than “chumash”, and “yontiff” for “yom tov”? It’s not, unless you’re simply used to using a distorted nonsensical pronunciation (“yuntiff”, minei ubeih, anyways makes no sense as you’re using two different pronunciations for the same “oh” vowel) and weren’t taught to say it right and are too lazy to change to the correct way. It is no more convenient than to say it right.
Incidentally, Mazel Tov is actually a “convenient” way of saying “mazal tov” (patach then kamatz).
So, while you are correct that you can, if you want to, for whatever reason, speak informally however it is convenient, the reality is that people are taught to, and still do, daven with these havaros.
Speaking of davening, there are people who say one or more of Hashem’s names with a cholam but other words with a Choylam. This is while davening, not while talking out of “convenience” informally.
Once again, why are people still being taught (to daven with) these questionable havaros?
“just my hapence”, I cannot imagine ever using “oy” instead of “oh”, in davening, no matter where I live. Why would you corrupt your davening with a made-up havara just because society does it? I understand the influence and being used to it, etc. but why don’t you speak to your Rav and convince others to join you in davening properly?