Nelly
All your concerns are perfectly normal - I certainly shared them before ds2 was born, when ds1 was exactly 2. I was lucky that I had a (2nd) very straightforward pregnancy so, although I was working full-time till a month before the end, I had (almost) the same energy levels to devote to ds1 so he didn't associate the arrival of ds2 with any change in service from his mum.
I agree that the 2nd baby could be totally different - I had a poor feeder/good sleeper followed by great feeder/awful sleeper. So you may be pleasantly surprised.
On the practical points you and Clara raise, I definitely agree that avoiding a heavy double buggy if at all possible is a fantastic boon. We've done really well with options of buggy board or baby in sling (one which allows you to feed) and I'm really pleased it's worked out for us. But it all depends how keen your toddler is on walking, not to mention how much public transport you get.
The more important decision is about nursery. My ds1 goes to my work nursery 2 days/wk and we decided that it was really important that he stay there for his continuity. So, although this means getting an early commuter train into London and going back again, he still has that aspect of his life intact and, when I go back to work, we'll have kept his place. So, if you're thinking of moving him, do it as soon as possible and make sure it's compatible with whatever you're doing about work after maternity leave. Might you be able to go down to 2 days instead of 2.5 and keep him there if he really likes it? Could dp help with the travel? My dh collects ds1 on one of the 2 days, which means I have a whole day with the baby once I'm home at about 9am instead of having to watch the clock in the afternoon. But you need to decide what will work best in your circumstances. And maybe a move to a local nursery would help to meet more local families, help with pre-/primary school applications?
Lots to consider and good luck with the rest of your pregnancy and the new life beyond.