In situation, when independence faction have every vassals in outside Greece also?
I'd probably still consider staying loyal (or "loyal", if you don't actually support the rule of your current liege...) to be a far more sensible idea, considering you'd be in a very reasonable position to take over the ERE (or further weaken it by forcing a shift in council laws, or deposing your liege in favour of a claimant) after the war is over and that that probably would be quite a bit more appealing than being an independent duke.
Maybe it would be worth considering independence if
everyone (between you and the border) on
every outer "layer" was part of the faction, but that'd be a nightmare to check for in script as there could be a ton of layers between you and the border (and presumably a one county wide line to the border wouldn't be sufficient if you're quite far away from it, so you'd have to check for a wide enough "slice", too), and considering you'd presumably be weaker than your former liege after independence it seems like a rather risky prospect to go independent when you hold de jure territory very close to the capital that is likely to result in you being de jure warred (or claim warred) into oblivion fairly quickly, so the smart thing to do would usually be to avoid going independent in that situation (at leat for the AI; a human player might have taken many steps to deal with reconquest attempts before even considering independence).
Besides, small-to-medium-sized realms (e.g. England, France, the Christian kingdoms in Iberia, and the Scandinavian kingdoms) should presumably stay together if they're culturally and religiously homogenous, as that'd be more true to history than them breaking apart at the seams the first time the top liege upsets his vassals, so a "You can always try to go independent if you're on the border or if a reasonable slice of everyone between you and the border want independence" condition would probably not work well without a "
unless you're a de jure vassal within distance X of the capital" caveat (with some possible caveats to
that, e.g. a realm with all laws (maybe excepting Succession Voting) in favour of the council, or that's unreformed pagan, or that's Early Feudal having a smaller X), and to cover most small-to-medium-sized historical realms you'd need an X that'd be noticeably larger than the distance between Constantinople and Nikaea.
At most, I'd be open to having a vanishingly small but non-zero (say 0.01x or smaller) chance of de jure duke-or-below vassals seeking independence from their same culture, same religion liege if they're closer than a reasonable distance X (as already mentioned, I'd probably go with X = 250 "distance units") if they're on the border/everyone outside them is in the faction, as there normally should be a reasonable amount of consolidation going on; the overall realm size distribution (in the relevant part of the world) you'd arrive at if you merged PUs and vassals into their liege's realm (as that's how those are handled in CK2) and also merged the HRE into a single blob (it CK2 terms, it would be a very council-favoured united realm) in the earliest EU4 starts and then projected that onto CK2 should presumably be the "average" CK2 outcome without player intervention, after all.