Yed desktop is one oldest tools I constantly come back to.

But as for OPs question, I would try any tool (diagrams.net or mermaid/graphviz ) and then when the diagram is clear, useful and makes sense, I'd go to graphics team your company use and make them unified with rest of the design.

Or if it's out of budget, go with excalidraw as it basically became current fashion and allows to easily build your own library of elements.

But still, advise for my former self would be still: break your process into two parts, where you focus on logic and visual clarity separately. So... Mermaid -> Excalidraw.