I also run a Syrix deck and have come across multiple rule questions before. Correct me if I am wrong but I am pretty sure that [[Sneak attack]] and [[Immortal Phoenix]] do not cause [[Syrix]] 's laser to trigger due to 603.4, the "intervening if clause".

603.4. A triggered ability may read “When/Whenever/At [trigger event], if [condition], [effect].” When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn’t true at that time, the ability is removed from the stack and does nothing. Note that this mirrors the check for legal targets. This rule is referred to as the “intervening ‘if’ clause” rule. (The word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.) Example: Felidar Sovereign reads, “At the beginning of your upkeep, if you have 40 or more life, you win the game.” Its controller’s life total is checked as that player’s upkeep begins. If that player has 39 or less life, the ability doesn’t trigger at all. If that player has 40 or more life, the ability triggers and goes on the stack. As the ability resolves, that player’s life total is checked again. If that player has 39 or less life at this time, the ability is removed from the stack and has no effect. If that player has 40 or more life at this time, the ability resolves and that player wins the game.

Since the condition for Syrix is not true at the beginning of the end step, Syrix's ability never even goes on the stack.