After enabling Gmail Keyboard Shortcuts you can also select all unread conversations in one fell swoop with *+u (which is really a 3-key press SHIFT+8+U, since I don't know of any keyboards where the asterisk is a lowercase character). Then you can use the previously mentioned keyboard shortcut to mark them all as read.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore