Hi,
In your Subscription -- Access control (IAM) blade, please click add role assignment -- click Privileged administrator roles tab, select Owner role, Next, select your account, Next, select Allow user to assign all roles (highly privileged), Next, Review + assign.
After making above assignment, double-check using View my access button to make sure it shows Owner role. I would leave Service Administrator in place and allow Azure to remove automatically later.
Another thing to consider is creating a separate break glass/emergency account that has been assigned Owner role to the subscription, Global Admin in Entra ID, plus in Cost Management and Billing -- Access control (IAM) assign it Owner to the billing account(s).
Preferable to have this account not subject to any kind of MFA, super-long password, excluded from any Conditional Access, and keep username/password for it in a safe or similar location.
The idea is if lose access via your main account (e.g. something happens to your phone and you can't respond to MFA using Authenticator) you have a way to restore access.
Article below provides more details on break glass/emergency access account I described above:
Manage emergency access accounts in Microsoft Entra ID
https://learn.microsoft.com/en-us/entra/identity/role-based-access-control/security-emergency-access
Please click Accept Answer and upvote if the above was helpful. If something is unclear let me know in a comment.
Thanks.
-TP