What is the ruling on my breaking my fast in Ramadan due to a legitimate Sharia-recognized excuse, while intentionally not making up the missed days and sometimes not praying?

You must repent for intentionally not making up what you missed in Ramadan, and you must make up all those days, as they are a debt upon you. Along with making up the days, you are also required to feed one poor person for each day you delayed making it up until the next Ramadan began, unless you were ignorant of the impermissibility of the delay, in which case no expiation is due from you.

As for the prayers you missed, you must repent to Allah for this sin, and the majority of scholars hold that it is obligatory to make up all those prayers.