The life-cycle emissions from nuclear are better than PV, but it’s still not as good as wind or hydro. But the issue is that it’s massively front loaded - you have huge emissions during construction that are slowly undone over the decades of operation. But we can’t afford to ramp up emissions for the next 14+ years (both the emissions of building a nuclear plant, and the fact that the existing coal/gas plants will have to run for another 14 years). If you switch to renewables, you can reduce emissions this year, not in the 2050s.
And there is absolutely no way you’re going to repurpose a fission plant into a fusion plant. They have basically nothing in common apart from the name.