Dominic, migration from amazon ec2 works fine with Proxmox 6.4.
I did the follow steps:
Generate vmdk image from amazon cli / moved to s3;
Download the vmdk image to Proxmox 6.4;
Create VM in Proxmox;
Detach / delete disk from VM on Proxmox;
Imported the image with qm importdisk ID ec2-ami-bbb.vmdkexport-ami-vmdk local-lvm;
Attached the disk to VM;
Add serial port to VM to boot correctly;
Ajusted cloudinit to not slow down the boot process.
I hope it helps.