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What I've noticed about selenium-toned Kallitypes is that they lose their brown tones and move much more towards black & white, some looking quite similar to silver gelatin prints.
That's odd; most of the time when you selenium tone prints like Kallitypes or Van Dykes, the result is a weaker/lighter brown and loss of dmax. Having the look shift towards that of silver gelatin I would actually consider as an advantage. Can you show an example of one of your prints where this happened?
Also, AFAIK there's no fundamental difference in archival stability between a Van Dyke and a Kallitype print. They're both rather sensitive to deterioration, but either process can be performed in a reasonably stable fashion provided the attention is spent on processing for archival purposes. Especially sufficient washing and indeed toning in a noble metal like gold as
@retina_restoration suggests, or alternatively platinum or palladium. Gold is more common and generally more accessible; I personally also find it the prettier one of the bunch given the pleasant hues it can produce, ranging from a rich warm/red brown to a cool, almost-neutral somewhat purple hue.