With the Seleucid Empire already large enough and difficult to administrate, either Antiochus III or his successor may wish to ease their own burden by just granting Egypt their own Basileus from within their own family. Better to have Egypt as a client-kingdom than another satrapy which the Syrian-based Seleucids would have to pay for its defence and administration.
On the other hand, Egypt is so damn wealthy, it can pay for it's own defence many times over and still provide a profit. IMO, the Seleucids, should they take Egypt, would rather sacrifice the soon-to-fall eastern satrapies rather then give it up. I mean, I took them generations of bloody conflict to take the place, and now they would just give it up and be back to square 1 ? Because any ruler of Egypt will have enough force projection capabilities to threaten Syria and intervene in dynastic disputes, civil wars, launch invasions etc.
I think there are three possible ways to achieve an independent Seleucid Egypt in the short term:
1. A member of the Seleucid family takes control of Egypt during a period of instability, but fails to take over Syria and/or Mesopotamia
2. It results out of Roman intervention in internal Seleucid politics, maybe as part of a "divide et impera" policy
3. The Seleucid Empire loses most of its other possessions (Persia, Mesopotamia, what they have left of Anatolia, maybe even Syria) and shifts its focus to Egypt