You are describing my periods!
Currently pregnant with 2nd and loving not having periods. Really want to get some sort of contraception after baby to stop them for about 10 years as I too have endo - mine is in my rectal area, somewhere called pouch of douglas. I get dreadful backache with periods. And yes, only diagnosed with an operation - and then removed with same operation of which I have had three.
However, as a suggestion for the heavy periods and flooding, have you considered a moon cup? Forget the mother earth types, I got one free and gave it a go and it is much better than tampax for the flooding. It does leak sometimes, but I just wear a panty liner for those episodes and a thin normal pad at night.
It is a bit weird putting it in and out at first, you need to be 'Friendly with your fanny' !
But it is a million times better than tampons and you have that freedom feeling away from pads.
They cost about £20 in boots, (bottom shelf tucked away from all tampax and big brands) there are two brands, one is called moon cup and one is called something else and comes in two sizes ! One for those who haven't had children and one for those who have.
Give it a whirl, the amount you are probably spending already on taxmpax it won't make too much of a dent.