I'm just adding a link to our most recent (July 2024) Implementation Methodology because I can't find it on our website(!).
https://drive.google.com/file/d/1YOcbLB76QRMbyD1m3AIEt-ICSeCys1pf
In general we don't favor BRD's due to them not being read by many people, and having a low cost/benefit ratio.
We have an ROI Analysis Template on page 64 which is a simple spreadsheet of requirements.
Page 66 has an example specification of the kind we favor.
As tempting as it may be to spend time on what is needed "tomorrow" it is far better to focus on what is needed "today" (and if they don't have it today they don't need it!) because the best time to decide how things would work best with Odoo is when the day to day Users of Odoo are proficient with the system - which is NOT until at least 3 months after they start using it.