How about looking for another nanny or enrol your son to infant care (if finance is a problem)? Actually I feel that your bring home salary 800+ is just nice to cover the nanny fee and your still can survive. So even you stop wking for awhile, you are just stop bringing home whatever you have to pay the nanny. if you intend to continue wking, i think can start to source for infant care or nanny. But since you said you wish to be a stay at home mum to tc your son and is able to work out on the finance part, then I think shdnt be a pro. =)
Every baby is different, not all will sleep thru at few mths old. mine sleep thru the night from 3 mths old onwards but I heard some only start to sleep thru at 6mths or 1yo.. It is recommended to introduce semi solid when the baby turns 6 mths old. Give him stage 2 formula only when he reach 6 month old, dont mix stage 1 and 2. you can just change it or replace stage 1 with 2 slowly.
As for the sarong, it is good to wean him off now. I only wean my son off sarong when he was 11mth old. Though he only sleep in sarong during daytime but it was still quite difficult. what i did was: let him fall aslp in sarong 1st then carry him out and let him sleep on the bed/mattress.. after a few weeks, i started to pat him on our bed. At 1st he will cry very badly but as time goes by, the cries shorten and stopped. after awhile he will know his routine.
Hope that helps =)