First of all, the meagre 'search' in 'Manage Bookmarks' does not tell you where a 'found' bookmark IS, which makes it next to useless. (If ONLY it would tell you that in the list you see when you click the URL.)

Over the years (on DESKTOP, I can only guess the horrors on tinyscreen) I've developed a system of folders with generic names that I use to sort BMs as I add them. My 3 top categories - the only ones visible in the 'Toolbar' are OFTEN (frequently visited sites grouped by folder), RESOURCES (folders at the top are most-visited) and LOCAL (most-visited on top). I also use the 'New bookmark order' extension, which adds new bookmarks to THE TOP of whatever folder I put them in (easy to open and drag-into folder topic).

Works, but it's hardly ideal, that's for sure. Don't think anyone at Moz has addressed this design in years.)