Bookkeeping CRUD app

Honestly with you: I doubt one can just sit down and write descent bookkeeping web-app alone and within reasonable timeframe. It is a task for year(s) considering ability to work full-time. You will have to write a lot of business logic, that comes on top of all that user-management, PDF generation, form builders, etc. Unit-testing will also require additional efforts, as well as security, API, overall architecture, several rounds of refactoring...

Back to your question about merging those software packages. "Pro Login" is CI3, "PDO Crud" is not CI at all, and most probably you would like to use CI4 in your project to benefit from long-term framework support. In short: this will be a pain.