I have a very similar history (5th round of ivf brought us our first fetal pole and heartbeat, early nhs scans due to ectopic and MMC). At 5+5 there wasn’t yet a visible heartbeat on TV scan, and CRL was measuring behind. At 6+5 an abdominal scan showed only 1.4 mm of growth since previous scan, and behind by about a week, but clear hb. The next abdominal scan at 8+5 measured 4/5 days behind, but then she got me to empty my bladder halfway, scanned again, and I was only 2 days behind. Every scan since then has measured spot on for my ivf dates (20+5 today and babe was measuring well all over at 20 week scan). I think no matter the stories you collect you will be worrying, I think I found literally every historic story here and Reddit of measuring behind during that worrying time (I was bleeding too). But it is absolutely true that when they are teeny, it is incredibly easy to measure “wrong” but be right. Heartbeat is most important.
I’ll add, I’ve started having my cervix measured weekly with TV scans, and even something as big as that (compared to a teeny fetal pole) gets measured like 5 times in a row all with slightly different results, so that the result is an average of a few measurements.
Lastly, know your name from here so im sure you’re as aware as i am, that private scans aren’t necessarily as accurate as EPU ones, certainly one of mine the picture we got was very fuzzy at 12 weeks compared to the nhs one (baby wouldn’t cooperate at nhs one for NT measurement). In case that causes extra worry. I hope you’re as lucky with your EPU as I am, they gave me fortnightly scans until 12 weeks. Will be thinking of you.