First, you can't say that any of this propulsion tech is remotely mission-ready. It's all very speculative. There's been no real-world testing of any kind. You'd need to at least test-fire it in orbit and prove a solar sail in particular. Any kind of nuclear propulsion adds whole new levels of proof-of-solution (yes I know RTGs exist but those are technically quite simple being just radioactive decay rather than something utilizing fission or fusion).
Second, it's not clear what kind of speed this could reach. At New Horizons speed, assuming you can find the right launch window, you're looking at 18-25 years transit. That's a long time for a probe to survive.
If you do adopt a solar sail, what happens to it over 20+ years? What happens from long-term damage of hitting dust and micrometeors? Could you need to course corret if it gives uneven thrust?
And all this for... a flyby. Obviously Sedna is too far and too slow for anything else. Just like Pluto.
But if we're talking 2j0-30 year missions, I'd rather send an orbiter to Uranus. About 20 years is I believe the time frame for an orbital insertion to Uranus. IIRC Neptune is closer to 30.