Why should it be shirk?
Tajweed simply speaking is the best style of reciting the Qur'an. Note that according the riwaya different tajweed rulings apply "tafkhim" isn't part of the naturally speech, even if you may find some muadhins in the Maghreb states doing tafkhim on the laam of "Hayya 'ala as-Salaat" based on the influence of riwayt Warsh 'an Nafi'.
Usually the tafkhim in case of the word Allah الله in natural speech is so light that it would make no difference for the listener whether you spell it more like "Allah" or "Alloh", while the first is more likely the choice when reading it outside the qur'an.
I'm no expert in tajweed, but I strongly assume that there are enough cases where the correct spelling sounds more like "Allah". But not that "Allaa" is rather wrong as the "h" is clearly pronounced!
So all you have is a confusion based on what you've learnt and you feel you've committing an error in the daily practice. This is rather normal as the more you learn the more you will question what you've done in past.