At 1g constant acceleration, including deceleration, it's only 9 years. A flyby is only 5 years [1]

[1] https://www.omnicalculator.com/physics/space-travel

Where would you get the energy to accelerate constantly for nine years?

That's the time passing in the vessel, not bad but still it means that such a mission would have to be completely self-sufficient and not rely on anything on/from Earth…

Not necessarily, if we could reach significant fractions of light speed. The subjective travel time can be shorter than 100 years, and approaches zero as you approach the speed of light. (Photons don’t experience time, so to speak, they are everywhere on their trajectory at once.)

As your target speed approaches the speed of light, the energy required aproaches infinity

Of course. But it’s a common misconception that traveling a distance of N light years will take at least N years subjectively. At something like 70% of light speed the subjective time already drops below the distance number: https://physics.stackexchange.com/questions/109776/how-long-...

suppose we have the tech to travel at a significant fraction of light speed, how long will it take for the spacecraft to reach that speed and similarly how long will it take to slow down when you are approaching a target planet?

If only wormholes stopped being "theoretical structures"

Haven't you heard? Wormholes are tired, Alcubierre drives is where it's at nowadays. All we need now is a way to shovel away that pesky dark matter.

Oh, that problem has been solved.

Haven't you heard? Dark matter doesn't exist [1]

Off to the stars!

[1] https://scitechdaily.com/dark-matter-may-not-exist-these-phy...