Unless you are streaming Full HD video or downloading/uploading large files, 6 Mbps would likely have been enough for your described usage.
People usually want higher speeds for media; e.g video in 4k resolution.
15 Mbps is likely more than you need. Not that it matters, beyond you paying a little more. But yes, the 1200 baht option would have suited your purpose.
To put the 100 GB thing in perspective....Imagine that a good quality Full HD movie is 3 GB; using the 15 Mbps AIS Marathon, you would be able to watch one movie a day and nothing else. But you could download the movie in twenty seven minutes.
With an unlimited 6 Mbps connection, it would take sixty seven minutes to download the movie, but you could download twenty movies in a twenty four hour period.
Better for GPS maps as you don't have to be concerned about running out of data with a fixed limit. Not that you likely would, given your description of your usage.
I guess there may still be the option to cancel and re-order if you don't want to spend that much on an occasional use SIM.
As an aside, AIS also have a six month option of unlimited 8 Mbps and unlimited in-network calls for 950 baht.