Is "seorang" optional here?
Yes, it's optional here.
"seorang" is a classifier/counter and it's translated into EN with the indefinite article "a" .
I think it's the same in EN, where you can also omit the indefinite article.
"My mother is a researcher" <==versus==> "My mother is researcher".
These ID sentences have the same meaning, they're all valid :
"Ibu saya adalah seorang peneliti."
"Ibu saya adalah peneliti."
"Ibu saya seorang peneliti."
"Ibu saya peneliti."
Super helpful about "seorang"! Thanks! That's exactly what I clicked on the discussion to find out about. One reply about English: I don't treat the "a" as optional in these constructions. I, and I think most native speakers, would always say "My mother is a researcher." Never "My mother is researcher. "