The trigger
A spreadsheet becomes a software problem when multiple people need conflicting access, process state is encoded in formatting, approvals live in chat, or one person becomes the only one who understands the formula chain.
Do not rebuild Excel badly
The first internal tool should model workflow, roles and state transitions rather than reproducing every column and macro in a prettier interface.
Build the smallest durable system
Start with the process that creates the most coordination cost and give it a reliable source of truth, permissions and observable state.