I don't see that you have. P2 is merely an assertion.
The entirety of a syllogism is a mere series of assertions; but I can elaborate. I have been thinking about this more since the creation of the OP, so my explanation might veer off a bit therefrom.
An immoral act is evaluated relative to the (1) the object(s), (2) the end(s), and (3) the dignity of the offended party (parties) involved. The order of importance is 3, 2, and 1; and 3 is a subtype of 1.
EDIT: it isn't really coherent to say 3 is a subtype of 1, I should have called 1 "the means".
3 holds the most weight, when considering punishment, because the thing offended (i.e., the object of the offense) is what is owed justice and punishment is about justice; and, so, punishment must firstly evaluate the value of the thing offended to determine how severe the punishment should be. E.g., skinning a rabbit alive is not as bad as skinning a human alive, disobeying the wishes of a random fellow-citizen is not as bad as disobeying a court order by a judge, etc. The objects and ends (1 and 2) could be identical in every regard and the dignity of offended party could cause the punishment to vary significantly.
2 holds the second-most weight because, after determining
what was offended, what was intended by the culpable party is what is most closely tied to culpability itself (since morality is about right and wrong behavior as it relates to rational deliberation—to willing freely through thought which necessarily is determinable through ends that one had in store for the act). E.g., a person that runs over a kid because they were distracted with their phone hasn’t done something as bad as running over that same kid purposefully even thought the kid has the same dignity in each scenario. Likewise, the objects involved (1) are tied to the ends (and intentionality) of course, but their could be a divergence; and what was intended is how we investigate the act since an act is a volition of will.
The last aspect, which holds the least weight, is the objects involved as it relates to the means. E.g., a person that murders someone else by way of murdering someone else to get their body to fall on the other person’s body (to kill them)(perhaps they shoot someone on top of a building so that they fall 1,000 FT onto the target victim thereby murdering both for the sake of murdering the one) is doing something worse than someone who just, ceteris paribus, murdered the same target victim because a part of the means was bad.
A sin is just an immoral act that has as one of its offended parties God. A sin, therefore, has at least two offended parties: the object of the sin (as the object of the act) and God (as the perfectly good being which wills the perfect order to things).
For the latter, a being with infinite dignity has been offended and this part of the sin does not fall prey to my OP’s argument; however, the punishments varying by objects or/and ends would have to be distinguished in varying by something—I am not sure what that would necessarily look like. E.g., having the end of killing this tree for no reason other than to go against God’s will is worse than killing the same tree in the same manner but self-gratification: both are against a being of infinite dignity, but they have different weights in terms of the ends one had; likewise, disrupting God’s will by being mean to someone in a relatively trivial manner (so to speak) is not as bad as disrupting God’s will by murdering someone: both are against a being of infinite dignity, but they have different objects (and ends, but I am emphasizing objects here) and dignities of those objects which were offended. Does this mean that it would be proportionate for God, if He did not forgive someone for their sin as it relates to offending Him, to infinitely punish them with some kind of infliction? Maybe: I don’t know.
For the former, no object of the act can have infinite dignity because it is a contingent being and none of them have been of infinite repetition (historically); however, to your point, it is in principle possible that the universe continues for infinite time and that some sins which are not rectified would “spill out” infinitely. If there’s nothing infinite about the act or its consequences, then it cannot be proportionate to punish the person responsible for the act with something infinite because something infinite is disproportionate to something finite.