I'm kind of the same, you have to be knowledgeable about the vehicles and do your homework first like you said.
I always have a vehicle that I want with the online paperwork, but, several others as much like it as possible from other dealers w/ that paperwork. I'm NEVER so in love w/ any one that I have to get that particular one either. When the salesman comes running out the door and ask if they can help me, I tell them only if they can negotiate w/ me, otherwise I tell them thanks but no thanks and go to the sales manager that usually is sitting in some elevated throne position on the showroom floor. I tell him which one I want and tell them he has one shot at a price, no offer/counter offer crap. If its good, I'll accept it. If not, I'm out the door to "go see one of these" and show them my stack of alternatives. Get a bottom line out the door price too that includes EVERYTHING like doc fees, window VIN etching, warranty's, floor mats, extra keys, etc, everything. If the number you get from the business manager is any different from the sales managers, tell him its wrong and he needs to redo it. The last 3 vehicles I bought were done like this w/ no bickering, dickering, etc. It was quick, painless, and I will definitely go back to them again.
You have to be able to say no and walk, otherwise they have you by the nuts. I had a manager tell me at the old Blackwell Chevrolet in Jackson "Mr Peeper, I don't think you really want to buy a vehicle". I got up and walked out with him following me the whole way apologizing. I went across town to Rogers Dabbs and bought one there. After picking it up next day I drove straight back to Blackwell and went inside. That manager saw me and said, ahh Mr Peeper I figured you'd be back. I said yes you were right, I wanted to show you the vehicle I just bought across town because they did business w/ me like I requested. I pointed it out to him and turned around and walked out for the 2nd and last time.
Never, ever give them your real phone number. You will forever get calls from them as well as extended warranty offers from dozens of companies if you do. I have a burner number that does work w/ wifi, but I don't ever use it for anything. Give them one like that you can get from the Apple store or Google Play.
If you are financing, go in w/ a letter from a credit union in your folder basically stating that you are pre-approved for financing. My credit union is always better than any rate they can offer unless its one of their 0% deals but even then, their vehicle price is typically loaded w/ the finance charge they are giving up. When dealer asks if you are financing tell them no its cash, because, in essence it is. They will be given a check from the credit union so its like cash to them. My credit union goes as low as 1.49% - 1.99% now on anything less than 66 months if your credit is A+. If its not, sorry.....In my opinion you shouldn't go more than 12-24 months anyway.
If they ask if you are trading anything in, tell them you haven't decided. I usually do trade but never until I've agreed on the new vehicle. I can't stand for people to come look at a vehicle I'm selling. They don't show up, they are late, they want to use your bathroom, they say crap like this is the LE edition, I thought yours was the XL edition. No, the photos and description clearly show what it is. Its not worth my time to go through that and then have them say they only have a thousand less than your "Firm Price". Good Luck!