Can I batch-migrate an existing 50k-image catalogue?

Yes. Media Optimisation ships CLI tools specifically for that. Point the worker at your pub/media directory, set concurrency (usually 4-8 workers on a typical 4-vCPU VPS), and let it run overnight. A 50,000-image catalogue typically finishes in 6-10 hours. The originals are preserved, WebP/AVIF derivatives are generated alongside, so you can roll back per-image if quality issues arise.