Have you checked out SdFat? It may be the 'older lightweight' library you are mentioning, but now it's a bit less lightweight but fully supports FAT32 etc.
You should be able to utilize the 32K flash space fully, what typically causes crashes is too many global variables in the 2K memory. Especially when using an SD card, which often needs a 512 byte buffer.
What may make it much easier is to log the raw 9 axis data rather than the calculated fusion data, if it fits your application- then you can focus on actually logging the data.