Anon… Please learn the following clear sources before making such statements, as they are demonstrably incorrect and might lead others to sin.
Eating before shacharis: gemara brochos 10a, based on the pasuk lo lochlo al hadam – it is assur to eat before praying for your blood.
Shu”a orach chayim – 89, 3-4.
There is a shailoh if it’s deoraysoh or only derabonon, but an asmachta.
It’s definitely not a “minhag”