If bloat is the problem
Rank Math loads a broad module set by default. Disabling unused modules helps; if you want a genuinely small footprint, The SEO Framework or SEOPress Free are lighter. Measure the difference with a real field test rather than assuming.
If multi-site cost is the problem
Per-site plugin licensing gets expensive across a portfolio. An external auditing platform prices per account and crawls any site by URL - including non-WordPress properties - which is usually cheaper than licensing a plugin ten times.
If you need to know why traffic dropped
This is the real gap. Diagnosing a decline needs crawl history, index-coverage trends, rank history by location and device, and content-decay detection. A plugin holds none of that history, because it only ever sees the current state of one page.
Schema: plugin output versus validation
Rank Math writes schema well, but writing and validating are different jobs. Google's required fields change, and an invalid property silently removes a rich result. Validate the rendered output on real URLs rather than trusting that the plugin setting is still correct.
How to run both cleanly
Keep the plugin as the output layer, disable modules you never use, and put audits, monitoring and rank tracking outside WordPress. Never run two SEO plugins simultaneously - duplicate canonicals and duplicate schema are a common and avoidable self-inflicted problem.