Incremental coexistence or atomic cutover: controlling cascade order during styled-components migration

Migrating a small application to styled-components without downtime forces a choice between incremental coexistence with legacy CSS and an atomic cutover. The concrete constraint is cascade order: styled-components appends rules to the document head at runtime, so during coexistence the final source order and specificity relative to legacy stylesheets are not guaranteed by the library. Across code-split chunks and SSR hydration, the ordering of injected rules relative to existing <link> or <style> tags remains an unresolved behavior that can make overrides unpredictable.

One documented approach is using StyleSheetManager with a custom target to isolate styled-components output into a separate DOM node, yielding deterministic ordering at the cost of extra SSR plumbing and potential FOUC if the target is not ready before paint. Another is relying on &&& or !important to force precedence temporarily, though this trades correctness for a hack that is difficult to unwind once the legacy stylesheet is removed.

Which trade-off is more maintainable for a small codebase: deterministic DOM isolation via StyleSheetManager, or accepting runtime insertion order with temporary specificity overrides during a phased migration? How should teams verify actual cascade behavior in their specific bundler and SSR setup before committing to either path?