Migrate to sql 2017 and add ag's
My instinct is to (side by side) migrate as is. Then once live and stable, separately, add always-on ag's.


That is perfectly reasonable and supported.

However.

Are you sure you really want to perform the cluster creation, configuration and AG provisioning and testing on a live (post-migration) system?  

If you do the AGs first, you can perform all that configuration and testing as many times as you want before the migration.