FWIW, the 26" Howa produced groups during load development that were similar to what you described. Less than ideal conditions that day, but the wind didn't create these "patterns" we saw on paper.
The owner was unaffected by it. Said that it took about 150 rounds for the barrel to settle down.
He suggested you try the 140 Berger hybrid target over h4350 and the 140eldm you've already developed. I know he was loading the bergers at 2.9 COAL.
Don't know how far north you are, but my range has access to 1200meters with plate racks at 500,550,600,700,800,900,1k, 1100, and 1200. Moving plates at 300, 500, and soon to be 700. If you're ever in the NWTN area, shoot me an IM and I'll get you out there.