Thin File - car loan

I have been building my credit and I finally put my loan application in with my credit union. I just got off the phone with the branch manager and she said my credit was great at 760 - she pulled my Equifax. She said that due to having a thin file, and no previous car loans I would only be approved with a co-signer. I don’t have an option and honestly prefer not to have a co-signer. I have $1500 for a down payment too.

She recommended I go through a dealership and six months after I purchase I would be approved to refinance through my credit union.

How high do you think my APR will be? I’m so worried and got my credit from 520 to 760 in six months… I worked so incredibly hard and thought I was doing everything right to get a great APR. I feel blind sided.

Anyone who has been through this can you give me some guidance? I’m pretty weary of this process and feel defeated currently.