Why do you need "Ego" if sum is "I am"?
You don't need it. 'Marcus sum' is natural and correct.
Ego sum Marcus/Marcus ego sum, etc. would stress the 'I'.
Marcus sum - I am Marcus
Ego Marcus sum - I am Marcus
Indeed, you do not need it, only if you stress it ( like: I am Marcus, you are Brutus’)
Ego sum Marcus...
...sed ego Marcellus ero