On the basis of personal experience, I haven’t found Strata Managers to be of much benefit IF you can find a Proprietor who’s interested enough and prepared to learn enough to manage the Plan’s affairs; this Forum is a font of excellence with regard to the latter!
Not all Plan’s require the same breadth of strata knowledge and a committed Proprietor can, with the assistance of the Executive Committee, save your Plan thousands of dollars in Management Fees, Disbursements, and operations / maintenance costs – not because all Strata Managers are uncommitted, but moreso because a committed Proprietor has only one Plan to worry about; their own!
However if you do want to terminate your Plan’s Agency Agreement with its current Strata Manager (SM) and engage another one, you first need to check the termination clause in that Agreement where you’ll likely find that you need to provide the current SM with 3 months written notice after a General Meeting of Owners has resolved to support the change.
As for strata management ratings — I don’t know of any, but I’m sure that an internet search would provide sufficient numbers of SMs from whom proposals could be sought after you’ve scoped what services your Plan requires.