If you're solo, fewer apps is better. Glo Invoice gives a freelancer everything to bill: estimates, invoices, recurring billing, branded PDFs, online payments, and customer records.
Pair it with a notes app for project briefs and a calendar for scheduling. For taxes, the built-in P&L and mileage tracker hand off cleanly to a CPA.
Avoid stitching five tools together — every handoff is a place data goes missing.