The reason voyagers went so fast and so far was the particular arrangement of planets in the solar systems at launch time. A number of maneuvers and gravitational slingshots were used. It may not be possible for a number of years to do this sort of thing again. And without this, you’d need a much bigger rocket than is possible to build.
The Grand Tour alignment was less about being able to visit the outer planets, and more about being able to do it cheaply. One spacecraft was able to visit four planets! That's rare! If you're willing to launch four different probes to visit those four planets, it's not trivial to do, but it's not hard. (Jupiter's position basically sets your launch windows.) And just hurling something out into the black as fast as possible pretty much is trivial.
Voyager 1 only hit Jupiter and Saturn, and it barely got a speed boost from Saturn.
We can boost from Jupiter into deep space whenever we want. Pioneer 10 reached a similar speed just fine, but we don't talk about it a lot because it broke.
