My sense is that we might have reached the limits of what we can do in high-energy or fundamental physics without accessing energy levels or other extreme states that we currently can't access as they are beyond our capacity to generate.

From what I've read (not a professional physicist) string theory is not testable unless we can either examine a black hole or create particle accelerators the size of the Moon's orbit (at least). Many other proposed theories are similar.

There is some speculation that the hypothetical planet nine -- a 1-5 Earth mass planet predicted in the far outer solar system on the basis of the orbits of comets and Kuiper Belt / TNO objects -- could be a primordial black hole captured by the solar system. A black hole of that mass would be about the size of a marble to a golf ball, but would have 1-5g gravity at the distance of Earth's radius.

If such an object did exist it would be within space probe range, which would mean we could examine a black hole. That might get us un-stuck.

If we can't do something like that, maybe we should instead focus on other areas of physics that we can access and that have immense practical applications: superconductivity, condensed matter physics, plasmas / fusion, etc.