I found early on there was an increase, but in the last 2-3 weeks it has dropped off almost completely.

This i believe was helped along by her having a bout of gastro last week and not feeding at all.
But about 4 days ago we finished altogether and she weaned, as in her words "They aren't working anymore mum". Like you, we planned to make it to her 3rd birthday in November or Christmas - I wanted to make 3 years! lol Not to be.

Not sure about the morning sickness - I had none with Charlotte and only some very mild queasiness this time around.
The only time I have had morning sickness I was pregnant with twins and miscarried - so contrary to popular belief, morning sickness is not a good omen for me! lol

Try and hang with it, your supply essentially is still demand driven - the more he feeds, the more you will produce theoretically and there are many women on BB who have very successful BF relationships with older children whilst pregnant and go on to successfully tandem feed.
Good luck! If you want to continue, keep going, hopefully your supply will catch up sooner rather than later!