Hi Max,
Your approach is correct, take a backup of your 3CX System which you will restore on self-hosted instance and also make sure to request the deletion of the hosted by3CX instance by opening a support ticket. (Note that if you login to the customer portal as the end-user, you can go to My Subscriptions, click at Hosted Tab and click STOP.)
Login to your Customer Portal (portal.3cx.com), go to ‘Subscriptions’ which is located on the left side of the portal and then click the subscription you would like to self-host.
Then, you can click the ‘REINSTALL’ button on the top right of the pop-up window, and select ‘Self-host in your Cloud’.
Click ‘Next’ to complete the setup until the step where you can choose your preferred hosting provider to complete the setup of your 3CX System.
For further information please check our guides for all the supported Cloud Providers here:
https://www.3cx.com/partners/hosted-pbx-providers/
After the deployment of the self-hosted machine, upload your backup to an FTP, sFTP or Google Bucket Storage space and connect this from the 3CX Management Console in the "Backup & Restore" section. Once connected, you can restore your existing backup.