Hitting the ALT key in Chrome, if you are anywhere *but* in the address/search box, will throw focus to the Chrome menu button, which you must then down arrow to expand.
Hitting ALT if you are actually in the address/search box does nothing.
Just curious, but have you ever tried the Chrome Bookmarks Manager, which you can open with CTRL+SHIFT+O? You start in the Bookmarks search box. If you tab twice you are in the tree view for your bookmarks that's analogous to the same thing in File Explorer or an e-mail client, but you can't use single letter navigation. If you know what your existing bookmark folder structure is, and you're looking for a folder, you can enter the folder name (or part of it) in the search box, hit Enter to run the search, then tab 4 times to land in that folder's list of links. You can, of course, also do the same thing for the bookmark name for a bookmark in the search box, tab 4 times, and be in the list of individual bookmarks (and, possibly folders if one matches) that match the search.
I find using the bookmarks manager in Chrome, and also in Firefox (where it conveniently hangs around in its own window named Library that you can ALT+Tab to when you want to revisit it) the easiest way to access bookmarks with the least amount of "brute force" running through the list looking for something.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.
~ Mark Twain