Your timing is pretty good: you want to fly in March 2020, and the AmEx devaluation will happen in mid -April 2019. Read this Point Hacks article about when award flights will become available. As you can see, you should be able to find award seats for March next year in March and early April this year, giving you time – just! – to transfer your points to the relevant FF programme from AmEx before the devaluation.
And that, in a nutshell, would be my strategy: wait to see what award availability there is, and then transfer your points to the relevant programme. My suggestions would be that if you want to travel on a Oneworld airline such as Qantas or Japan Airlines, transfer your points to Asia Miles. If you want to travel on ANA (Star Alliance) transfer your points to KrisFlyer or Thai (I’d probably choose KrisFlyer because the points take less time to transfer over from AmEx).
You can start doing some groundwork now by doing some searches for next Jan/Feb, seeing what’s available, how many seats tend to be released on each flight, etc … so you’re ready to pounce if and when you find availability on the dates you want to travel.
What you don’t want to do is transfer your points too early – wait until you know about availability, and then transfer.
All the best!