[Quoth]

I see your difficulty.

Search on Kobo is good because it looks at title, author, subtitle, series and collections.

I only use collections to find the most recently added book in that genre. I don't know how many collections I have but I felt at the start that using tags was a mistake (mysteriously the default for the Sony PRS350, so I changed that to Collection column.

I probably don't have as many as 50... I'll check. Hmm, more than I thought, about 100. But there are a number of stupid ones that have crept in over the years with 1 or 2 books.
510 on SFold, SF 116, Romance 250, School 197, Reference 73, Mystery 237, Fantasy 293, Plays 86 (obviously items in that should also be in other collections).
8069 ebooks (likely 75% PD, used to be 95%, but I went a bit mad this last two years buying ebooks, more than previous 10 years).

I think less than 50 collections matter.