I too went from multiple monitors down to one (from 3 to one) for focus. But I have a question. You complained that your 3440x1440 monitor was too wide, then you went to a 4k monitor? I don’t know the resolution of the one you got, but most are around or more than 3440 pixels wide.
So in what way did the 4k monitor help you?
I also wish I shared your experience that 4k reduces fatigue. For me, looking at 4k at any size under 30" strains my eyes due to how dense it is. Though I do love 4k at the 40"+ sizes