1) Used gpu prices seem to be very good vs new ones (although i have no experience with a used one). powervgx seems to be a reliable supplier here.
2) For 3080, can easily reduce power limits by 10-30%. Combined with undervolting, there is no perf loss at -10% so that's free and will only lose around 10% perf at 30% lower power. So that can be done to reduce power usage. Same probably applies to high end cpus as last stretch is generally inefficient.
3) DLSS 3 only improves visual smoothness from what i have heard, input lag remains same. Plus there are still reports of artifact issues. Best case scenario for DLSS3 seems to be above say 90-120 fps, so for lower tier gpus its impact probably will be lower. So maybe wont extend life as DLSS2 might do. Even DLSS 2 can be blurry at lower resolution depending on game. In control it was blurry at 1080p (quality). I have heard that generally it works best at 4k.
4) Maybe i got lucky ( or does it not affect 3080?) but my memory temps on 3080 Zotac are fine. In low 90s in stress tests ( superposition), lower in games and never 100+ as others report. So its not a given, and can maybe check if buying used.
5) Both 3080 and 4070ti have lower vram. Its fine now, but may become an issue in future. I saw that with my old 1050 2gb. So unfortunately tradeoff is between that and worse ray tracing performance on amd which has more vram in similar tier gpus. Worst case may have to lower texture quality or similar.