I think these are all normal (good!) things to think about. I have had similar thoughts. It's also true that there is no guarantee that your kids will be close/lifelong friends, although even if they're not they'll have someone with a shared upbringing, which I think is helpful in life.
What if my son doesn't cope.
Obviously it will be an adjustment, but I think there are ways to help prepare him. Lots of talking about what's going to happen, lots of reassurance about how much you love him, trying to get him involved as much as possible, spending one on one time after baby is born. Most kids adjust ok, so there's no reason to think your son won't (unless you see something specific that makes you suspect he might find it especially hard?)
What if there is something medically wrong with my new baby.
Always a risk. It was a risk with your first, too. There is additional testing you can do if you need reassurance/if you are sure you would tfmr (but it still won't 100% guarantee that there's nothing wrong).
What if I can't be a good mother to my son because I've spread myself too thin.
Billions have done it. You'll work things out.
What if it's twins.
Not hugely likely unless you have risk factors (older mother, IVF, twins run in the family through the maternal line (i.e. your mum/sister/gran had twins).
I didn't enjoy my first pregnancy, what if I don't enjoy my second. (I had a lot of morning sickness).
I also don't enjoy pregnancy. It's a means to an end. No guarantees that your first pregnancy will be the same as your first, but equally it might be worse. Definitely something to consider.
It's all a bit of a gamble, OP!