Your lining at stims day 1 is 5mm? That seems very promising!
I am doing my first fresh cycle at KKH and going for my BT tomorrow.
I think first time is always a trial and error because even with stats and experience, the doc still has to guess how you respond to meds. Eg I have a relatively low AMH at 1.6, doc gave me quite a high stims shot at menopur 300 iu a day but I still yielded only 7 eggs (5 mature, 4 fertilised). Now with this info, he can increase my stims dosage for next cycle since I respond not as well.
I also have a BFP pee stick but don't dare to get my hopes up until after beta tests, scans and first trimester. My DH and I don't dare to believe our BFP yet.
So, I guess the motto is hope for the best and prepare for the worst - I hope you yield many good quality embryos and baby dust!