First: The lines of terrain that extend from an adjacent "grand hex" when doing barony-scale terrain maps are probably better represented as wedges or triangles intruding on the primary terrain. The d8 still represents a length in hexes of terrain that extends from the edge or corner where the adjacent grand hex connects, but it also represents the width of the base of the wedge. It's an equilateral triangle with sides d8 hexes long.
Second: Instead of a wedge, you could do a corner or half-hex. Consider rolling 2d12 for one intrusion, instead of two. Reading from left to right, draw a line from the first clock direction to the second. Everything from the clockwise part of the grand hex takes its base terrain type from adjacent grand hexes. For example, if you roll 2 and 4, reading left to right, the clockwise part is a small eastern area defined by a chord and an arc from 2 to 4; if the left/right order is 4 and 2 instead, the clockwise part is the large, western area. This eliminates the need for the d8s, but reduces the terrain variety.
Third: The twisty lines technique is good for rivers, roads, and shorelines, but it might actually be easier to postpone adding twisty details to the edges of other terrain boundaries, like hex sides. Instead, assume that a terrain line/blob or hex boundary is only a rough sketch for the time being; when travelers move adjacent to a hex bordering on a terrain change, roll a d12 to see which hex it actually takes after. This allows for more ragged edges, and some spots of mixed-terrain you might not get otherwise.