its a common problem with catalyzed product,, they cant really offer slower cat ,, the only solution is to mix bigger batches and yes,, waste some,,
however that said,, there is a risk of undercatalyzed product,, its difficult to mix accuratly small batchs,, and you must insure getting complete mixing or there is a potential for non cured areas,,
I have resorted at times to refriderating the product to slow the cure time,, it does make it stiffer and harder to work with, but its an option