Trying this with my advent yarn. But each triangle will need to be smaller as I’ve 24x10g rather than 12x20g.

Cast on 86
Triangle back to 80 to start

Nope, too much leftover. Trying again with a cast on of 96 and triangles at 90.

Cast on an extra 14 stitches on the second wedge. Think this will work :o)

Using a marker to save too much counting!

On first row with a new colour count 14 for each wedge so far, then 6 and place the marker. This should leave 76 stitches still to knit after it. Knit to the end and cast on 14 stitches (now 90 stitches up to the marker).

Knit up to the marker. Remove marker. Wrap and turn the next stitch.
Knit 6 stitches and place marker. Knit to the end.

Repeat those two rows until you have only 6 stitches left in the wedge and then follow pattern for the lace row.

I started running out of yarn for the wedges so I shrunk them slightly towards the end. Wrapping at 7 stitches rather than 6 to take a couple of rows out.

Did the final cast off with the leftovers on day 1 and had barely 2 inches left! Phew! :o)