I would stick with the basic setup: 1 partition on the card, with one FAT32 filesystem in that partition.
Normally the card would already be partioned that way so you would not have to change that. Just create the filestem like you have below except with the partition number: mkfs.vfat /dev/sdX1 where X varies.
What commands you use depends on your distribution; I use cfdisk for partitioning and mkfs.vfat to make filesystems.
There is a recent discussion here about what size filesystem or SD card the GPSMAP will take. Garmin only says "32 GB maximum filesystem size", but doesn't specifically say whether the GPSMAP takes SDXC cards (> 32GB). I don't have any such cards to test with but some people are claiming they do work. I find it hard to believe a recent device would not accept SDXC cards, but the filesystem size limitation might be real.
(I tried making 2 partitions and filesystems on a card, and the GPSMAP ignores the second, which is not unexpected.)