Well, grab is escape ends, sustain minor... so perhaps add the following continuations, both standard actions.
Restrain - You may restrain a grabbed target with a Str attack vs Fortitude. (Escape still ends)
Pin - You may pin a restrained target with a Str attack vs. Fortitude. (Escape ends pin, but not restrain). A pinned creature cannot make any attacks except close attacks.
This ignores the fundamental problems with Grab/Escape.
1) Grab is a standard action, escape is a move action. So, it is faster to escape than grab (it can be done twice in a round, not once). That increases the odds of escape success by ~+5 (i.e. 2 40% chance attempts = a 64% chance of success, 2 50% chance attempts = a 75% chance of success, 2 60% chance attempts = a 84% chance of success, etc.).
2) Escape is a Skill check (from one of two skills) vs. a Defense (which should never occur in the game system since it is mathematically unbalanced). In other words, a skilled character gets +5 more than an average defense per attempt. So, it is easier to escape than grab.
3) Escape can be accomplished with push, pull, or slide of an ally with no rolls involved.
4) Even when a monster does not have Acrobatics or Athletics as a skill, they often have either a high Str or a high Dex. If it is a high Str, the Bigby's spells for example are terribly ineffective in game.
5) The grabber gives up a standard action attack in order to grab. The escapee can both attack and try to escape in the same round.
All in all, the Grab/Escape rules totally suck. Because of the problems listed above, it is super easy to escape and super difficult to keep someone grabbed.
Adding more rules for restrain/pin without fixing the fundamental problems with grab/escape adds a wart on top of sucky rules.
So with your rules above, it takes one round to grab the opponent, a second round to restrain him, and a third round to pin him. That gives the opponent 6 actions in which to try to escape. Even a cloddy Wizard can make the roll a few times in that scenario. One would never hold a Fighter or Ranger down with those rules.