I use Namecheap and sadly still Gandi for old domains.
The only issue I experience with Namecheap are included redirects which have something like 90% uptime.
Route53 domains is seriously not needed for anything - just add zone in AWS and point your registrar to new NS.
>Route53 domains is seriously not needed for anything
If you're already hosting on AWS, then you'd only have one potentially hostile company to deal with instead of 2.