This is one of the most useful applications of the theory of “negative harmony”.
The viidim chord in a major key reflects onto the iidim chord in a minor key. They both have equal pull towards their respective tonics.
Similarly, the V7 chord in major reflects onto ivadd6. So yea, from that perspective, the iidim and iv6 chords ARE the best penultimate chord in a minor key.
New here, not sure my notation is up to snuff. Would welcome input!