Hire an internal IT manager when the business has enough ongoing technology operations, suppliers, users and projects to require daily ownership. Use an external trusted advisor when the need is mainly periodic senior judgement: choosing systems, checking suppliers, planning investments or overseeing important projects. Some businesses need both, with clearly separated responsibilities.
Choose based on work, not title
Count recurring operational decisions, incidents, vendors, projects and governance tasks. If the work is continuous, it needs internal ownership.
External can be useful for independence
An advisor can challenge a supplier or major investment without being absorbed by day-to-day support.
Avoid a responsibility gap
Someone must still own accounts, backups, access, contracts, assets and incidents. External advice does not remove operational accountability.
How soapplied approaches the question
I would not start by assuming the stated problem is the whole problem. The first step is to understand the people, purpose, constraints and interactions around it, then test what intervention would improve the system rather than merely optimise one component.