If you have already attempted the self-serve account recovery methods, your next step is to move the domain to a new Cloudflare account. You will need access to change nameservers at the domain registrar and the transition will be smoothest of you create all of the necessary DNS records in advance. Some may be visible, but any records that are proxied will need to be created using the origin server IPs, not Cloudflare IPs. This will require that you know the addresses of the origin severs.
Once you have competed to move to a new Cloudflare account, ideally under the control of the domain owner, you can then invite your account to manage it. Be sure to secure all Cloudflare accounts with strong multifactor authentication, such as TOTP, a hardware key, or even both! Make sure you save some backup codes in a secure location when using MFA.