I am using a Teensy 3.2 which has a lot of resources to play with
start away from String class, not because you have resources, but because you don’t want problems later on.
try using strchr() like this:
char myData = "<0013A20040F7442C,100.134,0.254,0.0,0.0,99.1><0013A20040F7442C,100.134,0.254,0.0,0.0,99.1>";
int charCount = 0;
ptr = strchr(myData, '.');
while(ptr != NULL)
ptr = strchr(ptr + 1, '.');