Hello,
Interested in the project.
I am a Software Engineer specializing in end-to-end Web Development with expertise building solutions that leverage Google's GSuite platform.
Given the details you've shared thus far, its clear to me that your project is very broad in scope. The range of functionality you've described will not be trivial (nor quick) to implement. To build a multi-user application that leverages Google Sheets as an underlying database presents a number of challenges. For one, a spreadsheet is not an optimal alternative to a true database solution; not only is data retrieval and storage slower by several orders of magnitude but its query capabilities leave a lot to be desired. There are some workarounds for some of the issues that will crop up throughout the course of development but in a lot of cases there are going to be hard limits impacting performance.
Assuming you are content with the limitations of using a google sheet as a database, you require this web app to support trello-like features with integration with Google calendar and other components you have yet to mention. I'll be frank, going off those details, your stated budget strikes me as extremely impractical for a project of this scale. I suspect you've under-estimated the amount of effort involved. If you'd like, we can set up a time to talk and I can walk you through what can and cannot be achieved with this approach so you have a better idea of scope and cost.
Regards,
DimuDesigns