cdt_Lcpl: all sounds like good advice. Getting any background in with a Regt, especially OPs will help you.
Going to 7 or 29 will just make you fitter (probably - although you could be as fit in any Regt) and therefore more able to do the Selection for 4/73. The selection is tough and has PSIs from Hereford, but the Bty role as I understand it is "stay behind OP" - miles behind enemy lines, observing enemy movement, calling in MLRS and air strikes etc. More precise info is available on the links above that you have been pointed towards and other threads (?).
My advice would definitely be Phase 1, Phase 2, Regiment (which one is up to you, but OPs would be sensible, therefore Gun Regt) for a couple of years and then ask for 4/73 selection.
In my experience from Regimental Duty and Training Jobs, alot of "special" jobs within gunners (Para, Cdo, 4/73, 14
look really good from the other (ie. civvi) side of Ph1, but once you are in the system it can be one step too far to attempt straight out of the box. It would be a shame to attempt it too early and become disillusioned. BUT THAT IS ONLY MY OPINION - if you want it enough, go for it. If they are now recruiting from Ph2 then give it a whirl.