Is there a more political reason to stop sending weapons other than the seemingly unrelated reasons of "protecting domestic farmers"?
Yes. Putting aside that the weapons are still going to be sent (Poland is still a producer of arms and Ukraine can just buy it one way or another) and that this is mainly a verbal trick by the PM, there is. Elections and the fact that the Polish electorate's view of the issue is shifting, from decisive support for Ukrainian war effort (which was clearly the case when the "special operation" i.e. war escalation, happened) to being either indifferent (due to "war fatigue") or outright hostile to any mention of it and to Ukrainian people themselves (due to generally deteriorating economic conditions, low GDP, high inflation, instability, oversaturation of local job market with migrants, dropping quality of public services etc., for which the Ukrainians can be then easily blamed, either directly, due to migration, or indirectly, due to being engaged in a conflict that brought the crisis).
It's not a substantial shift yet, but visible enough to be played by the currently ruling far-right (PiS, Konfederacja) just before the elections.
An article from June 2023 on the matter (in Polish), still quite relevant: