I am answering with taking into account Legends, as there are not a lot of canon sources I know regarding this and you mentioned Legends sources (Darth Nihilus).
The studies of the Sith, since the introduction of the Rule of Two, were more covert compared to those you named. You named the feat of Darth Nihilus of consuming the force on a planet:
Nihilus was responsible for the devastation of the planet Katarr in 3952 BBY, killing and absorbing the Force energy of the Jedi at the Conclave on Katarr along with every other living thing on the world save one Miraluka woman named Visas Marr. Marr was collected from the surface and Nihilus began to train her as his Shadow Hand.
From the Fandom article on Darth Nihilus
One other remarkable feat of him: lifting a starship. From aforementioned article:
Nihilus could also use the Force to lift starships, as he did on Malachor V with the Ravager.
It is a comparable feat to Galen Marak, also known as Starkiller, who brought down a Star Destroyer:
As for Darth Nihilus feat of consuming the force of an entire planet, it was matched by Darth Plageuis feat of controlling life:
According to Darth Sidious, Darth Plagueis was a powerful Sith Lord who was able to use the Force to influence the midi-chlorians to create life, a power that was reportedly to be extremely rare for one sensitive to the Force to possess, and could even be used by certain Sith Lords to prolong or even cheat death. Utilizing this ability, Plagueis was able to conduct unnatural experimentations in his bid to discover immortality.
From the Fandom article on Darth Plageuis
So, the Sith did not become less powerful but more covert. As for Jedis, Luke's powers where comparable:
In 43.5 ABY, Skywalker learned the "lightning-rod" technique Ayna-seff, which he use to safely travel through the Kathol Rift. After spending time with the Aing-Tii, Luke was taught the Fold space ability which allowed him to teleport objects vast distances.
From the Fandom article on Luke Skywalker (Legends)
So, it's not that the Force became less potent and more that knowledge was forgotten.