All need deep infrared surveys at the ecliptic plane, but P9 needs a much deeper survey?
This question already discusses the challenges of discovering planet 9
The main problem of the hypothetical planet 9 as compared to Sedna and 2012 VP113 is:
Sedna and 2012 VP113 have orbits that reach far out but currently they are near their perihelion so it was possible to observe them in the visible light spectrum. Planet 9's orbit is far out or at least it is currently near its aphelion. Otherwise it probably would have already been discovered.
Also the planet's orbit is probably tilted towards the ecliptic and it moves very slowly compared to the stars in the background.
At this position the maximum of its radiation would be emitted in the far infrared and it moves before the background of the Milky Way.
So to summarize:
We would need a long term observation with an far-infrared capable, spaceborne, telescope (because our atmosphere absorbs far infrared radiation) which has to include the background of the Milky Way. The telescope has to be able to detect faint infrared sources. We could be lucky and planet 9 just happens to pass in front of a star as we are observing it. Then we could get our first hint with an optical telescope. But that's pure luck and we can't plan towards that.
WISE already tried to find planet 9 but did not find it. The James Webb Telescope may be our sole instrument capable of discovering planet 9 directly. But it's not built to permanently survey an area of the sky although it could confirm a finding.
Gaia could probably detect planet 9 if it happens to pass in front a star.
According to Phys.org's Scientists propose plan to determine if Planet Nine is a primordial black hole if Planet 9 were a primordial black hole it could be detected by the Vera C. Rubin Observatory by observations of occasional accretion flares from cometary debris.
But its a big if.
Dr. Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard, and Amir Siraj, a Harvard undergraduate student, have developed the new method to search for black holes in the outer solar system based on flares that result from the disruption of intercepted comets. The study suggests that the LSST has the capability to find black holes by observing for accretion flares resulting from the impact of small Oort cloud objects.
"In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole," said Siraj. "Once they melt, the small bodies are subject to tidal disruption by the black hole, followed by accretion from the tidally disrupted body onto the black hole." Loeb added, "Because black holes are intrinsically dark, the radiation that matter emits on its way to the mouth of the black hole is our only way to illuminate this dark environment."
This is discussed further in their arXiv preprint Searching for Black Holes in the Outer Solar System with LSST accepted for publication in Astrophysical Journal Letters.
Note that LSST now refers to the Legacy Survey of Space and Time, and the Vera C. Reuben Observatory used to be called the Large Synoptic Survey Telescope. What is the LSST now?