How do you manage WBS brain fatigue?

I think you got both the main strategies I use: sleep on it, then try sharing with stakeholders to get feedback.
I would not share with all stakeholders, maybe choose a small group or some peers you can talk through it. I find that if I can explain the plan to someone (or even to myself!), it means it makes sense.
I refer to it as a "3D picture": if looking at my "2D" MS project I can picture the flow of information and activities in my head, then I am on the right route.
Do not go crazy with details, you can always apply a rolling wave planning strategy to add details. Keep a list of potential activities to add on one side (a One note/Excel dump will be good enough) and review every few weeks/months to add details to the next stages.
Also remember people working with you on the project may not be pro MS project/Gantt chart users, so going over 3-4 levels in the WBS may not add value as your reporting to Project board is at very high level and daily work is very detailed. You may be better off using a task management tool to communicate with the team the detailed tasks each team member has to work on. Good luck!

I will cross-reference to Operational & Capability Views. Thanks!