Hey, my agency specializes in GMB review removals so I'll try to weigh in on this. Mass reporting the review is still the most effective solution but it's a lot harder to execute now since Google's latest update. Not only do you need a pretty large quantity of reports from unique accounts to reach the necessary threshold for removal but the accounts must also be reputable enough to have their reports carry some weight behind them.
A lot of service providers are still utilizing random aged accounts used to leave reviews for their report spamming but in the same way that their reviews rarely stick, their reports simply don't impact the standing of the review in any meaningful fashion. The accounts doing the reporting should ideally have an extensive history of contributions on the platform (other than just leaving reviews) and at the very minimum, have some points towards reaching the "Local Guide" status. The higher the local guide level of the reporting accounts, the fewer of them you'll need to execute a successful removal. Level 3 of the Local Guide status is where the reports begin to really have a solid effect. I can't speak on the exact number of reports you'll need from these accounts to successfully take down a review (as it's a dynamic variable that tends to fluctuate from one review to another) but anywhere betwen a dozen to a couple dozen is the industry standard.
