I haven’t tried going that far in one run. But I’ve used buses fairly often to get to other cities.

changing buses 2 times in 2 different non English speaking countries

That alone would make me not want to do it, unless you were in a country like Japan or the Netherlands with decent pubic transit and quality google maps support.

are travelers allowed to eat in the bus? Am I allowed to bring my own food?

You’re not supposed to eat meals on the bus. It doesn’t stop it from happening. It depends on the temperament of the driver. But don’t do it, the other passengers don’t wanna be suffering w/e smell on a long trip. But you can pack food and eat it at rest points. (if your route has them)

but did you ever do something like this? Any regrets?

My first time in a major city, I was too scared to ride the subway. So I took the city buses everywhere, I regret that. Changing buses in the rain, and discovering my ticket was non-transferable was a huge drag.

Just know if you do, the local scammers know the stops and will try shenanigans. Don’t fall for the guy having a heart attack trick like I did. Pay attention to how the locals and fellow travelers are behaving.