Thank you for your help, pazos - everything is much clearer now. I had already found that link before actually and tried to tweak the hardcap but without understanding what I was doing
My metadata.calibre file is 39.3MB, and changing the max_json_filesize to 40 * etc. doesn't work. The Kobo Libra 2's RAM is 512MB, so I'm not sure it's wise to try and increase that further. And you are correct in assuming that I'm an 'advanced' Calibre user
Right now, I'm not willing to change that (or how I send books to the device - I'm still using the Kobo original firmware alongside koreader) so I'm not sure there's much to be done. I'll keep using the regular file search in the meantime.
I would be willing to share the log if you think that's helpful.