Alt+Down Arrow drops open the full Chrome menu. If you immediately follow that by B it will open up the Bookmarks submenu which you can then Down Arrow (and/or up arrow once you're after entry number one) to go through them.
I have not tried first letter navigation in that submenu and don't have the time at the moment (nor do I have JAWS handy right now).
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back