Any advice or insights into flying long-haul with a toddler?

Hubby and I are deciding whether to take our toddler to Thailand this summer. She will be 22 months and we plan to have a second LO soon, so this will probably be our last opportunity to do this trip for a while. ETA - I think she will be fine once we are there - she's a pretty easy baby.

She's been on lots of flights and mostly done fine, including 9-hour flights to/from Europe last summer which were fine as well, but she was 9 months and barely mobile.

The flights are about 20-21 flying hours with a short layover (so 23-24 hours of travel). We may break it up with an overnight on the way back. We have the option to do 2 business class seats (LO sits/sleeps with one of us) or fly economy and buy her her own seat (with car seat). I know car seat is always safer, but I'm focusing on making this trip as easy as possible. Advice?