Hey everyone, finally registered to express my thanks for all the answers in the community - particularly in regards to the UConnect - and share my experience.
I have a 250TB SanDisk extreme portable SSD connected to the 12" via USB C (USB 3 on the back of the arm rest) - NTFS, mix of flac and mp3 files. Suddenly newly added files weren't showing and I remembered reading about a folder limit.
I have around 13,000 files in 1,060 folders (Category/Artist/Album/) on the drive but only around 11,000 were recognized by the UConnect. By only moving files from individual album folders into the parent folder (Artist/) and bringing the total number of folders to 999, the number of recognized files increased to around 12,500. The rest must be in limbo due to breaking naming conventions and having special characters or some such.
So it seems it's not the file format of the drive or the total number of files on it (let's see if this is still true once I cross 20,000) but the number of folders on the drive.
Hopefully this helps!