Funny, I ran into the same mess with old railway ties in my last place. Thought I could just clean them up and use them for a rustic border, but the creosote smell never really went away, no matter what I tried. Polyurethane didn’t stand a chance—just like you said, it peeled right off after a winter. Ended up swapping them out for some chunky limestone blocks. Heavy as anything, but at least they don’t surprise you with weird chemicals or splinters.
Cardboard’s hit or miss for me too. Sometimes it’s like magic mulch, other times the weeds just poke right through and mock your efforts. I guess there’s something to be said for putting in the extra work upfront—stone or brick might be a pain to move, but once it’s down, it actually looks better as it ages. Plus, you don’t have to worry about what’s leaching into your soil.
Limestone blocks are no joke to move, but yeah, they’re basically set-and-forget. I tried using old bricks once—thought I’d save a few bucks—but half of them crumbled after the first freeze. Guess there’s a reason people go for stone even if it’s a pain at first. As for cardboard, I swear the weeds in my yard treat it like an obstacle course... they just power right through. Sometimes you just gotta pick your battles and go with what’ll last.