It looks like you’ve solved this but here’s my 2 cents: The jagged line is a result of your texture resolution. The smaller the texture the more jagged and pixelated. So you could have kept the UVs how you had them initially and either A) scaled up the island to acquire more texel density or B) increase the texture resolution as a whole.
Though as you’ve found, cutting a UV seam in that location is another perfectly valid way to solve it. Good thinking.
