Ultrasound accuracy for dating?

How accurate are ultrasounds to determine due date/conception??

I’ve had about 5 and the early ones were right on target (I think anyways, they wrote the GA on the pictures but I don’t remember if they actually measured or just inputted the due date I gave them but they look similar to babies at that GA when I search for pictures) and the 19wk anatomy and 36wk growth were on track within 3/4 days depending on where they were measuring.

Can I trust that my due date is correct aka my date of conception should be when I think it is?? I don’t remember if I actually had my period when I thought I did or if maybe it was just implantation bleeding. I had my baby but he looks nothing like who I think is his dad.. and I think my mind is playing tricks on me because the ultrasounds wouldn’t add up to my ex being the father (baby looks just like my first son and even me as a baby). My newborn has blonde hair and a forehead/head similar to my ex. BUT both me and boyfriend were blonde as babies/toddlers.

Ex- had sex last on 2nd of month last I remember

I believe I had my period on the 7th of that month.

Had sex 2x the weekend of the 21st when I supposedly would of ovulated, he may have came inside. I don’t remember.

So if it was ex’s baby the ultrasounds would of shown at least a 2wk difference in the beginning right?? If the 2nd was implantation bleeding, I would of been 3/4wks ahead right?? I think I’m seriously just overthinking this way too hard.