The comment from Cloudways talked about wildcard SSL certificates but didn't comment about the Domain Management tab, so I'm adding a comment to help others who might come across this, like I did.

Firstly, setup the DNS wildcard to point to the IP. Then on the Domain Management screen you can add a wildcard subdomain in the additional domains list - e.g. *.mysite.com
This stops you from needing to enter details on the Domain Management screen every time you add a subdomain.

You still need to add each subdomain manually to the SSL Certificate screen though, if you have using a combination of subdomains and mapped domains. But if you are only using subdomains in your multisite with no mapped domains, then you can choose to use a Let's Encrypt Wildcard certificate.

Can't believe this is not a thing, cPanel has had this for like 10 years.

As Todd said (in new platform) when using a Custom Certificate, you can just generate a wildcard certificate (*.mydomain.com), that you can use for WP Multisites, or any other needs you have. Wildcard certificates are not yet supported via Let's Encrypt certificate authority.

Cloudways Team

You just gotta add *.yourdomain.com as an additional domain in the domain management panel and then you can add as many subs dynamically as you want.

Not just wildcard DNS for subdomains but for domains. It would be nice to use a multisite domain mapping plugin so that a user can enter their domain and it just works, without us having to manually enter the domain in the Cloudways dashboard.

Agreed - Manual method of adding sub domains is like going back in time. It's inefficient and counter productive. Cloudways is a fantastic platform but these things need to be sorted. This is quite a hindrance for me to move all my sites here.