I’m also American, learning Spanish, but my understanding is that “en serio” can be said as a question or a declaration, just like “for real.” Jose: For real? David: Yeah, for real. Jose: ¿En serio? David: Si, en serio. I also think “en serio” could be replaced with “de verdad.” Jose: ¿De verdad? David: Si, de verdad. I think….

Some people may say "te lo juro" as a filler word. For exemple: Me gané la loteria, te lo juro. Mi trabajo es muy aburrido, te lo juro. This is very specific but in some parts of Colombia, we say "a lo bien". "a lo bien estoy cansado"

Hola Adam, puedes decir "De verdad?" es muy similar y su traducción al ingles es la misma, "for real"

I think it depends on the country in Argentina for example it is posta in Mexico I thing it could be neta (im not sure since im not mexican)

Those two options are the only two things that come to my mind at the moment 😄