The technology for a Dyson sphere is primarily the technology for suspension bridges, solar farms, and intensive hydroponics (all well understood) + the technology for the ISS (well understood at this point). The only missing piece there is a fully sustainable (i.e. only energy input) life support system, which we have existence proofs for on Earth but haven't built to reliable engineering standards yet. If we have the material, we could start building it today and get designs that worked well quickly, even if we had to ship in life support maintenance alongside computer chips & pharmaceuticals.

The part we don't know how to do yet is getting the material. Building structures that large in space, the resources for it have to come from space, we cannot lift it off Earth. So we need to figure out how to mine asteroids, and maybe also how to mine Mercury. Either one would be sufficient at the start. Neither of those are a well-understood problem, let alone solved, so that's where you should invest resources if you're a billionaire looking to start a Dyson sphere and think SpaceX is on track for launch cost reduction.

I think in terms of science and engineering difficulty, it's a pretty even race as to which is more difficult between making fusion actually produce net useful energy and being able to mine & refine materials from an asteroid + build a sustainable life support system.

The reason fusion "seems easier" is because it is economically far more achievable, and there's a lot more political will behind it so it feels more possible.