You won't have any problem with any good quality 64GB microSD and there is no reason to think a 128GB card should cause any issues so long as you can format it to FAT32. Use a modern disk format tool such as the SD Association's formatter or a recent version of GParted and accept its automatic partition scheme and that you will "lose" maybe 5 or 10MB before and after the file system. Don't try to "reclaim" that space; it isn't wasted as it means the file system starts and finishes on notional cylinder boundaries, as intended. This matters for performance and reliability.
I don't think there is any limit in Rockbox to the number of files, or at least not a limit that you might actually encounter in the real world. The maximum number of files you can view in a single directory in the Rockbox file browser is 10000 (ten thousand). The maximum number of audio files that can be referenced in a single playlist is 32000. Yes, that is thirty two thousand. You should be OK
I don't know anything about lyric files. I am sure you can find out whatever you need to know in the manual i.e. http://download.rockbox.org/daily/ma...box-build.html