ERP programmes stall when the first release tries to replace every spreadsheet at once. A useful cut is the loop the business already runs every week: who raises the order, who commits stock, who invoices, and who looks at the exception.
For a stone yard that loop is concrete. A lot arrives, a slab is located, a quote holds it, dispatch cuts it, accounts invoice it. That is enough for a first release.
Name the records. Customer, slab, quote, order and user are enough to expose ownership, permissions and the reports that matter. Extra modules can attach later if the boundaries were drawn on purpose.
A Next.js frontend can carry the first operational UI. A Spring Boot service and PostgreSQL become the right next step when transactions, integrations and long-running jobs need a dedicated backend. The marketing website stays a separate deployment either way.