You can use your preferred email provider. You can create new profiles with manually entered credentials (“Use Classic Email as Relay” option) and it is possible to add a second relay this way as well by using dclogin:// URL. There is no built-in UI for adding non-chatmail second relay and the message does not mention it, but it is possible. There may be UI for this later, although it will likely cause problems like users logging in to the same mailbox with multiple profiles.
These same messages are in the Delta Chat in SQLite database. What do you do with the messages on the server? Do you have a particular usecase for archiving encrypted messages like this? I am not aware of an email client that can handle such amount of small messages and modern OpenPGP.
What does “retaining control over the messages” mean? Practically the easiest way to access the messages is using Delta Chat itself, you can backup SQLite database and build tools that use it.
Maybe focus on one particular usecase that is broken by using a separate IMAP inbox with its own address instead of a subfolder of the inbox used by another email client.