Hmm, another thought would be to just not invade India. Sunda aren't likely to try and invade Bactria - and it preserves Bactrias strength which should sufficient to resist any nomadic invasions, and prevent a civil war.
If neither event happens, we don't see the kingdom collapse.
Instead a better route would be to partition the Parthians between Bactria and the Selucids (assuming they have the strength - this I do not know).
If Bactria ensure they get a coastline, then not only do they have an ally through which greeks can immigrate, but also coastal towns that could enable safe immigration from Ptolematic Egypt, and Greece proper.
I'd expect such a partition to include Gedriosa and Aria - and grant them the Persian coastline east of the Strait of Hormuz.
So perhaps we need the King of Bactria not to be interested in the Sunda? or have Sunda lose a crucial battle and not take out the Mauryans? I remember reading somewhere that it was Sundas rise that brought the Greeks back to India.
So yeah, more tasteful than my earlier thought, but they'd still need to win at least two wars - to quell nomads, and against Parthia - all whilst keeping generals in line. Interesting times indeed.
Though Greco-Bactria would now be well placed to focus on Indian, Scythian, and Tarim/Urumqi schemes, I have to add a disclaimer that whilst it may be more hellenic (or fully hellenic) - I'd expect it to not benefit the greeks in the med - and that they'd eventually form a new Bactrian identity.