NEED to use temporaries for this, and pretty sure there's more than just two solutions, but I was able to find one very quickly by assuming symmetry and using the hint from Mica at the end.
By temporaries, I don't just mean "What happens if I fill in this square?" I mean "What happens if I negate this square?" because there aren't any longer numbers along the edges that you can actually prove anything with, but using temporaries to test negations on the rows with sixes, does help. Very tricky, not a particularly good hanjie, but solvable if you're looking for something VERY difficult.