I had a similar issue. However i dealt with it differently. I decided that if a single menu item had 20 or so sublinks then it warranted its own page/post. I then created a new post and listed the sublinks therein with some good SEO friendly filler text at the top of the page and a CTA at the bottom if any one new of any other good (sites) let me know in the comments below and I will add them to the list.
The other tip I would give you is that if you have that many sublinks to manage you should enter them into the Easy Affiliate Links plugin which helps you to easily manage and keep track of the relevant URLs and give them pretty SEO friendly permalinks. Don’t worry if they are not actually affiliate links. Adding a check for broken URLs function is also on the devs to do list which would also be another plus.
The other thing you could explore is using one of the mega menu plugins that would give you the space to display your list horizontally. Rumor has it that Tom is thinking about adding this feature to GP however he doesn’t have much user experience regarding the need or use of mega menus and so has been reticent to move forward.