RE: You cannot be a Jew and believe in deities outside of Judaism.
Do Jews who believe in Jesus believe in deities outside of Judaism?
Absolutely. They are basically forfeiting any right to identify themselves as a Jew if they claim to worship any deities outside of our one G-d.
Simply: yes. Jesus as diety = a diety outside Judaism. “The Lord is with us, the lord is one” Furthermore, 1000s of 1st century Jews were crucified by Romans. Locking onto one, then declaring him a diety, then blaming his crucifixion on Jewish leaders/ppl (scripturally, ahistorically), this cannot be overlooked.
Note: Jesus as a person who existed in history is widely known - both by historians & Jewish ppl.