Re: EX4600 recommanded frimware standard or service?
It's difficult to say - but overall it should be the same. "R1" are the earliest release so in generel R2 or R3 should include more bugfixes and therefore more stability. On top of this is service releases (S) which are pure bugfix releases for a specific release - eg. 17.3R3-S1.
As stated I would go with 17.3R3-S1 in your scenario as it's a new implementation so no need to start on 14.1X53.