I’m afraid there’s no short-term solution to your problem. As @jvanasco and @sahsanu said, it does sound like your domain probably meets the criteria for a Public Suffix, since you give out subdomains to arbitrary third parties. However, you should be aware of the limitations - if you ask to be put on the Public Suffix List and get added, you won’t be able to set cookies on your base domain (owpanel.net). However, best practice is to not rely on cookies on any domain where you give out subdomains, so hopefully you are currently not relying on owpanel.net cookies.
Keep in mind the rate limit resets once a week, and you can always renew for a group of names you’ve already issued for. So as long as you gradually issue for new subdomains, you can increase to quite a large number of supported subdomains over time.
Also, you can combine up to 100 subdomains on a single certificate. So with a 20 certificate per week limit, you can handle up to 2000 new subdomains per week.