Learn Farsi: به عنوان داور، فکر می‌کنم این سوپ لذیذ است، ولی یک کم چرب است. - As a judge, I think this soup is delicious, but it is a little greasy.

To emphasize the concession (meaning "It is delicious, but..."), you can change لذیذ است، ولی to لذیذ که هست، ولی. The second clause would become این سوپ لذیذ که هست، ولی یک کم چرب است. This topicalizes the positive trait (deliciousness) before softening the blow with the flaw.

We've learned a few ways to say "as" or "because". Why use به عنوان (be onvân-e) here?

به عنوان specifically translates to "in the capacity of" or "as a [role/title]". It's the standard, natural way to frame an opinion from a specific professional or personal standpoint. Using something like چون داور هستم (chon dâvar hastam) would change the nuance to "Because I am a judge".

The sentence uses the formal verb form است (ast), but also includes یک کم (yek kam). Is that a mix of formal and informal Farsi?

Yes, it is a slight mix. است is the standard written "is", whereas یک کم (a little / a bit) is quite conversational. In a strictly formal written text, you would likely see کمی (kami) or اندکی (andaki) instead. In everyday spoken Farsi, the whole sentence would shift and both instances of است would simply become ه (e).

Could I use فکر کنم (fekr konam) instead of فکر می‌کنم (fekr mikonam) here?

Yes, in spoken Farsi, the subjunctive is very commonly used to mean "I think" or "I guess". However, the indicative used here sounds slightly more deliberate and analytical, which perfectly fits the context of a judge giving a formal evaluation.