You can de-risk your overflow menu concept with either a tree test or a first click test.
Create some tasks where the participants have to find items that you've placed under the "More" category in your tree structure. Then you can measure the percentage who were able to successfully navigate to the right nodes, how many got there directly or indirectly (a.k.a. stumbling around) and how long it took them. You could even A/B test one tree structure with an overflow menu, and one without.
Assuming you have a large enough sample of participants representing your user population, if your task success rate for items under the "More" parent category is:
- Under 40%: Poor, Try another approach
- 40-60%: Fair, but low confidence
- 61-80%: Good, higher confidence
- 81-90%: Very good, high confidence
- 90% or more: Excellent, very high confidence
A word of caution with tree tests is that they're actually not great at predicting actual performance on a live site.
First click test
If you have a live site or mockups showing the overflow menu concept you can use, this might be a better option for your specific scenario, because it sounds like you're really focused on whether your users will be able to find items in that "More" category. So you could create tasks to find items that you've placed under the "More" category just like you would for a tree test, but you would only capture the first click for those tasks (heatmap) and how long it took participants.
If you don't see a high concentration of clicks on that "More" button or dropdown, and if it's taking users a long time before clicking, that could be a sign that you might want to explore a different solution.