I know this is a very old thread but I've only just aquired my 2011 Octavia with 6 CD changer Bolero unit and I have some SD Card info which may help others.
64 GB Card, FAT32 = NO ISSUES
128 GB Card, ExFAT = NOT RECOGNISED
128 GB Card, FAT32 = NO ISSUES (used dashcam software to format as FAT32)
The interesting thing on the 128 GB Card was that on loading my library, only the first 10,000 (ish) tracks show up. In my case the alphabetical listing ended at "Tina Turner" whereas the last album should have been "Yes".
On examination the Tina Turner album and all preceding added up to 10,250 (ish) tracks, when I removed albums and bought the total tracks to just under 10,000 tracks they all showed up ... RESULT ... and that's more than enough for me 😃
So summarising, it appears the limitation is not card size but total songs, limited to around 10,000.