I have the same issue with Windows 10 v1903. My 64GB USB stick won't format FAT32 without an external program. I was able to format a 16GB USB stick with FAT32. An 8 GB stick should work for the US as the download is 6.92GB. I'm installing to a 16 GB stick and will report back on the actual space used once the Garmin Express finishes writing the USB drive.
Also, the car was able to see and write to my 64GB USB drive formatted to FAT32 using large sectors. 64GB sticks require large sectors because there are too many sectors at the default of 512 bytes, which is why I suspect your iMac doesn't report FAT32 as an option.
I used Rufus 3.5 to format my 64GB drive. In your case I'd search for free USB drive formatters for your version of iOS.
2018 Honda Clarity Touring PHEV - Forest Green w/Tan interior (wife's car)
2017 Volt LT - Heather Gray; black bow ties, Charcoal VoltShelf
2012 Cruze ECO MT (hail totaled 5/8/17 103,600 miles @42.5 MPG)
2010 Mit Lancer GT MT (traded for ECO @31K miles)
2002 Pont Montana AWD - title to son at college graduation
1990 Pont Transport (traded for Montana @240K miles)
1986 Fiero GT MT (traded for Transport - needed more seats)
1985 Fiero 2M4 MT (traded for Fiero GT @8K miles)