Learn Spanish (Latin America): Estoy enfermo y, además, estoy cansado.

The word además is a neutral way to add information, like saying "in addition" or "besides." If you wanted to complain and emphasize that being tired makes the situation surprisingly worse, you would use encima (on top of that).

Why are there commas around además?

Just like in English, Spanish uses commas to set off transition words and connectors. When a connector like además comes right after the conjunction y (and), it is framed by commas to show a slight natural pause in speech.

Do I really need to repeat estoy in the second half of the sentence?

No, it is not strictly necessary. You could simply say Estoy enfermo y, además, cansado. Repeating estoy here just adds a little extra emphasis to make both statements feel equally important and heavy.

Quick refresher: could I use soy instead of estoy here?

No, you must use estoy because being sick and being tired are temporary physical conditions. If you use ser and say soy un enfermo, it changes the meaning entirely—it describes someone as a chronic invalid or even a "sick" (creepy) person!