Oh wow, 13kg is a big boy! Mine is only a few days off being 11 months and has just hit 10kg. He's also really tall, top of the chart but is tall and skinny. He's furniture walking and pushing his little walker around so hopefully he will be walking before I start getting a belly too. I feel for you with ms and looking after your DS. There is nothing worse than being sick and trying to look after a baby.
We were also about 4 years to get DS so also didn't want to wait too long before trying again in case it took a while to conceive again. I'm not too worried about the age gap. I'm getting DS into childcare hopefully 2 days a week so I can keep working this year and hopefully do enough to be eligible for the PPL again and will likely just keep him in next year. He's done a couple of trial days and loves the playtime with the other kids