I’m not totally new to unity3d nor noob but there are lots of in depth things that I don’t know.
Are there professionals that I can hire to ask them questions regarding the things that I don’t know and get their advice regarding my project?
Yes I know that I can in the forum for free but I have a bad experience in the past, amateurs were answering all kind of wrong answers.
Unity Premium Support provides access to a dedicated team of Unity experts. You can post your issues to this team using the Premium Support web portal…
Then you were asking on the wrong forum. Our forum is chock full of professionals and even the amateurs are very often just as knowledgeable with many of them working in related industries.
I’ll take payment for personal service. Might as well get paid for something I’m going to do anyway. 
Try job forums, if they’re still around. Some people would most likely willing to tutor you, if you’re paying.
Or Unity Connect if that doesn’t work, supposedly the same gist but better
(supposedly, I’ve never been on it to support or disregard the claim)
I saw there were some new Unity courses you can take online at Udemy.com. It’s kind of like an online University, and I think you can ask questions to the instructor. But I don’t think you get the real one on one time you might be seeking. It’s more of a “message back and forth” type of thing, like a forum. Maybe they have some courses that will cover what you want to know though.