Sumasen is likely regional. I've never heard it said, but it has the same root as sumimasen. In spoken "standard" japanese, you'll hear "sumimasen" or "suimasen", which is a common deformation because it's easier to pronounce. And indeed, its root is "to finish something" (済む), while its meaning is large, including "Sorry" and "Thank you" in english.

Arigatou, literally, means "difficult to be/have" (有り難う (arigatou) is an euphonic change (onbin/音便) of 有り難い (arigatai; ari/aru: be/have...; ~gatai: difficult), which translated like this is just completely weird, but so is literal translation.

Arigatou is fine in many more cases than you claim it to be, but yes, Sumimasen can be appropriate in many cases where you would say "Thank you" in english.