I cannot change the role of the only owner.

A business always needs one owner. Make someone else Owner first, then change the first owner.

A manager needs one extra permission, not a whole new role.

Create a custom role with just that permission and add it with Custom on their row. Custom roles are on the Team plan.

Every login has one system role that sets what they can see and do. Pick the smallest role that lets them do their job.

Owner: everything, including billing and closing the business.
Admin: everything except deleting the business.
Manager: bookings, customers, POS and payments, staff, reports, refunds and door access. No business settings.
Staff: front desk and cashier work: bookings, customers, POS, taking payment and checking members in. No reports across shifts, refunds or staff management.
Coach (gyms only): looks up and adds students, books sessions and records workouts. No billing, services or staff. Made for shared gym tablets.
Accountant: read only sales and finance reports, receipts and commissions. Cannot change anything and cannot use the POS.

Nothing changes for customers. The person sees a smaller or larger menu the next time they open the console.

I cannot change the role of the only owner.: A business always needs one owner. Make someone else Owner first, then change the first owner.
A manager needs one extra permission, not a whole new role.: Create a custom role with just that permission and add it with Custom on their row. Custom roles are on the Team plan.