The path for EX3300 is correct but for EX2300 and EX4600 you need to do two steps.
EX2300:
There are 5 releases between 15.1X53 and 19.1 (15.1X53, 18.1, 18.2, 18.3, 18.4 and 19.1), so upgrade path is something like:
15.1X53 -> 18.2 or 18.3 -> 19.1
EX4600:
There are 7 releases between 17.3 and 19.1 (17.3, 17.4, 18.1, 18.2, 18.3, 18.4 and 19.1), so upgrade path is something like:
17.3 -> 18.2 -> 19.1