There have been various tzadikim who’ve davened Shachris well after the normative zman, some even after chatzos. (They practiced the same principle with Mincha and Maariv.) They even did so with a minyan (i.e. they weren’t doing it alone.) What shitta(s) are they based on?
They think that if you start “preparing” for prayer at the correct time, it’s considered like you started davening on time, and once you start davening at the right time, you can continue for as long as you want without a problem.