Really? Doesn't the rule that says the Rogue gets their SA so long as they attack an enemy that is adjacent to to an ally mean that your ranged rogues should almost always get it just by firing at the enemies their melee party members are attacking? I mean, that's what has always happened at my tables.
Pretty much the same here. As long as the target has an enemy within 5 feet of them, SA is on the table, even for ranged attacks.
Of course, we play that making a ranged attack into a melee provides the target with
some cover (minimum +2 to AC). This also serves to represent the additional care you take
if the target's adjacent enemy
is actually you ally.
Overall, 90% of the time rogues get SA each round, either by shooting at a target engaged with an enemy (usually an ally to the rogue) or by engaging along side with an ally against the same target. Spells such as
Greater Invisibility, magic items, actually being hidden, also contribute to SA occurring more often than not--by a wide margin. We even have a Fighter (Samurai)/ Rogue (Assassin) who can use his
fighting spirit feature to give himself advantage if necessary, allowing him to still SA even if other factors haven't brought it online yet.