When I do jazz improvisation, there are 3 root keys I use to make up the minor chords and 4 keys to make up the major chords... I.E c, f, and g for the minor chords and c#, eb, ab, and Bb for the major chords.

Two of my favourite chords for transistioning to the major key to minor or vice versa is the sus 7 and the M7.

Sus 7 utilises all the root keys for the minor chords; c sus 7 uses c, g, and f.

M7 is a great chord to use when going from major to minor because it uses two minor and major root keys. I.e AbaM7 (Ab, C, Eb, and G)