Screenshot of your email DSN settings?
Looks like you are still trying to use SMTP credentials. You need to create and use SES API credentials.
Yes, looks correct. Please show content of your plugins folder (ls -al plugins/).
Did you clear Mautic cache after you installed that plugin?
php bin/console cache:clear
Current Mautic installation might be broken. Could you try reinstall Mautic (using .zip method from GitHub releases) and redo these steps (git clone into plugins folder etc)? And check if issue still persists.
If I reinstall the Mautic files, will I lose the segments and emails I’ve already created?
Yes, during installation it overwrites database.
If your current Mautic installation already has stuff (emails, segments etc), then I would install on different subdomain for testing, like marketing-test.domain.com, that way you can test if issue still persists or not. After that you can delete it.