Search work should be assessed against the advisory enquiries you wanted to attract. Operational work should be assessed against time recovered and the quality of the process. Compare equivalent periods around filing peaks, rather than treating a busy January as a neutral baseline. Recovered capacity only has financial value if the practice can put it to use.
Specialist support is worth considering when there is a defined constraint and someone in the practice can own the change. If the desired client mix is still undecided, settle that before commissioning a campaign. The brief should also reflect the practice's approved description of its specialisms, relevant professional standards and Making Tax Digital changes where these affect its clients.