I also recently moved my SM server to AWS and it has been working great. Maybe I am just lucky, but the elastic IP address I got was clean so am doing SMTP straight out (after following the amazon process for opening port 25).
I am running a `t3a.medium` EC2 instance with a bunch of ECB block storage devices (system, email, web, logs broken out onto different drives).
My costs have been about $100/month for about 100 domains, 250GB storage, 100K emails per month.
I also moved all my DNS onto AWS Route 53 which makes things slightly easier, and seems to work well. The only hiccup I had here was that I host a few domains that I am not able to update the authoritative NS servers for. In these cases, I set up a forwarding DNS server that gets requests from the internet and then asks route 53 for the answer. This works almost all the time, but in a few places in the world it does not and I can not figure out why. If you are a DNS master, please get in touch!
Here is some very rough (only needed it to work once!) code I wrote to automate most of the steps...
Probably not directly useful, but maybe a good starting point for someone facing a similar migration. Feel free to ask any questions!