You seem to have a problem with your clerk.
Supporting papers must be made available to Cllrs AND the public at least 3 clear days before a meeting.
Neither Cllrs, nor the public, can be expected to ask appropriate questions, present challenge and fulfil due diligence if they are not provided with the information well in advance of being asked (in the case of Cllrs) to cast a vote one way or the other.
The budget process is fundamentally in the public interest since it is the public that will be required to pay for it. There really are exceptionally limited justifications for conducting a budget process in confidential session and to do so is very likely an abuse of the very limited privilege of holding a closed session.
The November ICO newsletter included an open letter to public sector leaders to reenforce the importance of openness and accountability in public office: "...transparency is essential if the people and communities we serve are to have confidence in the way that public services are delivered..."
https://ico.org.uk/about-the-ico/media-centre/news-and-blogs/2024/03/information-commissioner-calls-for-senior-leaders-to-take-transparency-seriously/
It sounds like your process is completely the wrong way around - you have to examine the historic financial data, the future projections and the existing balances in order to arrive at the precept requirement.