Congratulations on your pregnancy!
There were 19mths between my 2 boys. I had BF my first boy and had a very positive experience with it all the way along. It was some thing that I wanted to do for my second child but had the same worries and concerns that you have mentioned...I needn't have worried!! He was happy to continue to play away with whatever he was engaged with before the feed started; or he went thru a phase of selecting some books and I'd read them to him while feeding DS2 (his books were stored right beside the chair I use for BFing, so it was easy for him to get more when we'd finish one book); or he went thru another phase where he loved to look at the pictures on my phone. To his credit, he is independent and made the adjustment very well to not being the "single & sole" Xx. Could you maybe work on helping LB to be more confident playing on his own between now and Dec? Then maybe before you are due to feed Baby, you could set LB up with an activity you know they'll be happily engaged in while you feed? Starting this now and gradually reducing your interaction/ support with him beaten now and Dec would be worthwhile.
As regards having to stop groups/ activities - that will only be for a very short time while you get on your feet and find your groove with Baby. It's funny how quickly we forget the various different stages
but none of them last particularly long before they have grown and developed and changed again.
Sorry I can't link on my ipad - check out the forum on "And then there were 2", as there are great threads and plenty of advice from all of us that asked the same questions/ had the same concerns heading into that big change
HTH