I have a 450 and love it.
I’m afraid the cache limit is not one of filesize, but of memory and is fixed, depending on the model.
“The Oregon 300 and 400 support up to a maximum of 2000 geocaches (in addition to the 1000 waypoint limit). The Oregon 450 and 550 support up to 5000 geocaches.”
The micro sd card is there to allow Garmin to sell the oregon with additional maps. Eg, if you buy the exhorbitantly overpriced landranger OS map for the garmin, it arrives on an sd card and you just slot in.