Spoiler: the focal point is 3.5x the distance to Voyager 1.
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it isn't realistic assumption. Until you're talking pure solar, the amount of acceleration is limited by the reaction mass available. Actually to get there in 10 years with the Isp 3500 3 stages are necessary, or better the Isp should be increased 2x-4x - still seems doable - to get with like 2 stages with realistic [today] parameters of the reactors/etc.
Maybe it has further missions in deep space after that. Or look in other directions and use other stars.
Compare to nuclear powered ion thruster. Say we get reactor plus generation at 5KW/kg total - takes some engineering, yet nothing unrealistic for current tech (even 10KW/kg seems pretty reachable). Reactor is on a long pole with only small protection wall directly between reactor and payload. Say 5 ton reactor, 25MW. 100 ton whole rocket, 80 ton of it reaction mass. At current NASA 40km/s ion trusters we get delta-v 80km/s in 60 days. If we get thrusters with 80km/s - wikipedia mentions that current ones reach 50km/s, so don't see why we can't increase voltage and thus ejection speed further - then it would take 240 days to reach delta-v 160km/s (i.e. current multi-year missions to Jupiter/etc. would get in well under a year, and it will be with like 10 ton payloads). Don't see solar sails coming close to that - https://en.wikipedia.org/wiki/Solar_sail#Inner_planets.
And as i mentioned earlier - let say we got thrusters with 400km/s. The same rocket will get to 800km/s - 1500 years to the nearest star - in 20 years. 3 stages - 500 years to the nearest star. 1 ton final payload if starting with 1000 ton rocket like the one described above.
Gathering reaction mass ram style - it needs big apparatus and needs to be efficient. Doesn't seem realistic with current tech, yet i'm sure will be on the table once the tech matures.
2. 1 ton is starting from 1000 ton rocket. The Saturn V and Startship are on the scale of 5000 ton and assembled on Earth. That interstellar rocket will be assembled in space anyway, so not being subject to any meaningful gravitational nor accelerational stresses, we can easily build a 100000 or even 500000 ton rocket - basically just the reactors and tanks of acceleration mass - and thus get 100-500 ton payload. If we get any [semi]hybernation going (may be combined with 3d printing or CRISPR-like repair of organs, whatever we get in 20-30 years) or more probably some brain [partial] upload integrated with AI into some capable cyborg, may be even some people or those merged human/AI cyborgs would be able to go.
And by collecting some additional reaction mass ram-style over that distance and time (as long as we have enough reactor power to use part of the collected mass to avoid slow down resulting from the collection) we'd probably be able to slow down some small probes to land and orbit various objects in the target star system.
Take into account much much harder radiation in interstellar space, which will require much heavier radiation shield. We can make as many circles around Sun as we need, like a commet.
say at the Mercury orbit we unfurled the large sail and got strong boost, and we'll come back for the next round in like 100 years. It is something we'd have to do if there weren't better alternatives. Nuclear or solar panels + ion thruster inside the Mars orbit, nuclear + ion thruster outside Mars seems to beat pure solar. Interstellar - nuclear + ion or my favorite Orion project seems to be again better than pure solar. It is like sailboats vs powerboats - while we love sailboats, the powerboats are really more practical in all the cases except for the lazy relaxed cruising.
>Take into account much much harder radiation in interstellar space, which will require much heavier radiation shield.
Until we have a way of getting like 0.1c, any interstellar takes hundreds of years and will be done either by pure robots or cyborgs and beside some shielding the main way of dealing with radiation damage is to catch/repair ECC memory style.
For 0.1c we have either project Orion - though nobody seems to be willing to go that way (we'll see how it goes once we have operations established on the Moon and Mars, may be somebody will turn to it as 1. they would have a business case for it and 2. it isn't really possible to do such experiments and development on Earth anymore) - or today it looks more like the fusion-exploding small pellets like NIF at Livermore does is the way to go. We can reasonably expect continuing improvement in the gain in those experiment, and while Earth based energy generation requires higher gain and efficient conversion of that small explosion into electric energy which is still a problem to be solved, the space drive application requires exactly such a small explosion, and thus i think such fusion drive will come much sooner than an Earth based fusion power station.