Why the 6 is on its last legs? I've just replaced my 6 for the 12, and I'm continuing using it for work, for messaging and big email account managing and is working pretty fine (I've changed the battery 2.5 years a go, 98% battery capacity). I guess that the difference 7/8 is substantially on cameras but if you say "A 7 would be absolutely fine..." so then go for it unless you get a good deal on the 8... Think that the 7 (and specially the 8) is noticeable thicker and heavier than the 6 even if they have the same form factor. For the future support, I think that the 7 will have another iOS release at minimum and the iphone 8 should have at least 2 extra ones minimum. But is difficult to predict. If you think in support as "service repairs" then you don't need to worry, I'm still able to get official repairs for the 6 nowadays)
If the 6 is in good general condition you can also consider in replacing the battery. A bad battery is not just affecting the battery life, is also affecting the performance of the phone dramatically.
I hope it helps!