Tech stack sounds fine honestly, Next.js can handle both your frontend and API routes/backend logic without issue, and it'll scale reasonably well for a SaaS like this. The real bottleneck is going to be the video model itself and how you handle async processing since ghost mannequin generation isn't instant, you'll want a job queue and webhook setup rather than blocking requests.

Worth checking if there's a pay-per-use option instead of committing to a subscription while you're still validating demand, saw one for Seedance 2.5 at https://klifgen.app/create-seedance-2-5 that does pay as you go, which seems smarter than locking into recurring costs before you know your usage patterns or conversion rate.