First, it sounds like you may be a little confused about the grapple turn order. The grapple cheat sheets help, but here's some clarifications:
- You Grapple the foe. This is a Standard Action, and you cannot activate Greater Grapple because it allows you to maintain the grapple with a Move Action.
- The enemy gets their attempt to get out of or take control of the Grapple, or simply attack you (bad idea since you're a snake, but it's there)
- Your goal turn, you get to Move Action Greater Grapple, Standard Action Grapple, and Swift action Rapid Grappler.
How do you get there?
First of all, there is nothing in Pathfinder that will ever allow you to accelerate your turn order to before your opponent. You or your allies can inflict conditions that prevent them from acting on their turn, though:
*Whether a Panicked character can attempt to break out of a grapple or if that constitutes forcing him to cower is up to the GM
I ordered these in the general likeliness of a Pathfinder adventuring party being able to inflict them. I left out other things such as sleeping, Petrified, etc that would make Grappling pointless.
Other than that, you have to look for ways to increase your effective CMD. CMD calculation is
10 + Base attack bonus + Strength modifier + Dexterity modifier + special size modifier + miscellaneous modifiers
and the Special size modifier is (for increasing it):
Medium +0, Large +1, Huge +2, Gargantuan +4, Colossal +8
Knowing this, the best ways to increase your CMD for situations your allies wont crowd control your target are:
*These options are also up to your GM. Availability of ADCT feat is up to them as it normally requires Bellflower Network reputation, and you may or may not change your creature type when you Wild Shape. If they consider you animal typed, you cannot be affected by Enlarge Person and you'd prepare Animal Growth, and need to have Natural Spell to cast it after your form changes.
Then there is the option of level dips:
Monk/Unchained (1): +Wis to CMD, requires armorless which is no problem since you won't need armor
Half-Orc/Halfling/Undine Favored Class 1/level
Maneuver Master Archetype (1) Not a CMD increase, but allows an extra Combat Maneuver when you could full attack at -2 to all checks that round
Halfling/Tengu Favored Class 1/level
Lore Warden Archetype (3) +2 CMB and CMD
Brawler (3) +1 CMB and CMD
Strangler Archetype (2) Don't lose Dex while grappling, effectively +2; added bonus Sneak Attack dice while grappling