I think that there's a mindset among younger coders that "if it's not a modern post-AWS cloud provider, servers will take ages to come online and aren't going to give me full access, that's why EC2 exists." And this is conflated with the myth that running a mail server is hard.
But in practice, you can find any number of VPS providers, running in local datacenters, with modern self-service interfaces, with at least some IPs that aren't already spam flagged (and you can usually file a ticket to get a new IP if you need it), that are often cheaper per month than AWS, and give full root and everything. Find a service that will help you warm the IPs before you send to customers, and you're good to go!
But in practice, you can find any number of VPS providers, running in local datacenters, with modern self-service interfaces, with at least some IPs that aren't already spam flagged (and you can usually file a ticket to get a new IP if you need it), that are often cheaper per month than AWS, and give full root and everything. Find a service that will help you warm the IPs before you send to customers, and you're good to go!