When I was shopping springs the only Tanabe springs that showed to fit the 2018 6 were the NF210's which is what I installed and yes, the NF210's 2014+ will work on the 2018.
Why the TNF173? I would double check as I'm not convinced they will fit. I'm not saying they won't - just didn't show that they would when I was researching 4 months a go.
I think we have a bit of confusion here, the NF210 is the spring series, TNF173 is the specific part number that fits all 2014-2018 Mazda 6 models so you guys are talking about the same thing.
I too have the Tanabe NF210 TNF173 springs installed on my 18 Mazda 6 and it was a exact fit. The only thing that would concern me with the wagon is that the rear end is a bit heavier than our sedans, it may sag a bit back there with the rear spring rates being softer than the fronts. But who knows, the weight is likely negligible and won't make a visual difference.