A SaaS is not a site with a login: it is a multi-tenant product with billing, roles, APIs and observability. Every choice made at the start costs ten times more if changed later.
The MVP must solve a real problem for a real user. The risk is not doing too little, but building things nobody will use.
Recurring billing, webhooks and error handling are not details: they are the heart of the business. Better to design them before the first paying customer arrives.