Currently, all bookmark access is geared towards mousing. The fastest keyboard-only access to launching a specific bookmark I have found so far, is ⌘⌥B, ⌘F, [type search], ↹, ↹, ↓, ↵. Thats... not great. Using the URL/Search bar, Firefox has a clever prefix (*) for searching bookmarks, among other things. Orion already shows bookmarks from the URL bar:

With the * idea, typing * welc would have only showed the bookmark, like performing a search in the Edit Bookmarks window.
Although mentioned in this post, the OP actual meant bookmark keywords, which Orion already has, so I thought it worth opening a separate suggestion.
Other current solution I could find (but havent tried yet) is a Raycast Extension. Not a fan of this but maybe some would find this useful in the meantime.