A wedding-planning spreadsheet is the most useful thing you can build in the first month, and it costs nothing. A good one tracks the budget line by line, the guest list with RSVP status, the vendor list with contract and payment status, and the timeline. Four tabs, one document. No fancy formulas required.
Below is the structure, set up to be copied, customised and filled in. You can build it in Google Sheets, Excel, Numbers, or Notion; the structure is the same either way.