We are taught that we must not allow 3 days to pass without the study of Torah. Because of this we hold the Torah reading service on Shacharit, Monday and Thursday, and on Shabbat Morn' (all of course if we have a minyan). I do not see in the sources whether there is a prohibition for having daily Torah services if a minyan exists. Is the limit on Shabbat, Rosh Chodesh, Hagim and Mondays and Thursdays a minhag, or is there some point in halacha I am missing?

I think that, of course just to read from the Torah is no problem. the problem is making the Brachos on the Torah. SInce they only decreed reading on those times, so they only decreed making a Bracha on those times. Brachos on any other time would be L'vatalah.