Awww, OP, you are totally normal and all that is entirely natural! I have been there!
But I would say when it happens you will barely think about it.
My DH has two children from his first marriage. We didn't actually plan our DS for long - we had only been dating about 5 months when BF (at the time) told me how much I would love a baby, and how we should do it (embarrassing and immature I accept). My coil needed changing, didn't replace it, and one month later I was pregnant. The story is more complex than that, but not relevant here - and has all ended very happily!
In terms of your specific concern - many times when I was pregnant the fact that (now) DH had seen it all before (twice) was helpful. If he pitched it carefully in a non-judgey way I could accept and value his advice.
Sometimes he would NOT pitch his advice carefully, and then I can assure you I told him fast! Either I would snap that 'don't patronise me, not my fault this is so boring for you', or 'Jesus, can you just let me discover it for myself as I haven't done this before!', or 'maybe I would like to do it differently this time!'.
I was always semi-careful to bark at him in a good humoured way, he always knew what I meant and backed off.
On the small stuff - his son and daughter's first names start with the same letter. I always knew there is NO WAY I would use that letter! We actually have used the same hospital (well known private London one - I still fancied it even tho he had been there before).
I must say I never ever think about this now. Our DS is now 2.5 and absolutely besotted with his fantastic half brother and sister - we literally cannot have them with us enough to satisfy him! And they love him so much too. We are a happy family and I am so grateful that my DS has siblings to enjoy fun times and live with. DH and I are equally learning how to parent all three of them at the various stages in life they have now reached.
As my DSD and DSS are now 12 and 9 I recently realised that the age gap is a bit big to be genuinely full time family (and they don't live with us - we have them every other weekend, plus about 6 holiday weeks through the year). So, HO HUM, here I am nearly 8 weeks pregnant trying to grow my son's beloved family for him. I despise being pregnant and don't especially like the newborn stage.
I never even think about he fact that DH was a parent before me now. Seriously - I am just so happy he is the father of my child(ren).
I totally understand how you feel before he event, but if he is the man for you it would be SO sad to deny yourself that happiness because of some potential insecurity which I doubt will be an issue. When you are doing it you don't have much time to worry about whether DH has done it before!