I just had a friend come home early from her trip to Bali with a 12 month old. She struggled with the hygiene factor, but was off the beaten track.

Are you staying in a resort?

I have travelled through Thailand with a 3 year old and 9 month old and used the carrier a lot, but did like having a stroller for meal times/shopping days so bub could sleep. It is hot for both mum and bub to be carried always.
We just had a crappy one from home we took. We thought we would just dump it if it was a pain but actually used it a lot, including putting 3 year old down for naps if bub was carried. I would think a 21 month old would need a day sleep and you will appreciate having somewhere to put them, even if it means you're by the pool and you can both relax knowing DS is with you and safe and resting/snoozing/eating etc.

ETA you can hire strollers over there if you need and have it delivered to your hotel so it's there when you arrive. We did that while Travelling in USA. With a child 21 months though I'd want a stroller in the airport too though and would consider taking one with me.

They really aren't that hard to travel with, but carrying sleeping child and suitcases is tough.