We aren’t given much reason to believe that the stones are less powerful together.
Once Thanos obtains the Power Stone, he doesn’t use it to destroy a planet. However, he does use it in concert with the other stones for something much more impressive. See below.
He also uses it to destroy the massive Asgardian ship, and probably Knowhere.
We aren’t given any reason to think he couldn’t destroy a planet with it, though. It just never comes up.
As far as why
The energy output of the Power Stone against individual characters seems lower.
Keep in mind that it could be working at a maximum power density. Enough to kill every ordinary person on a planet if they were exposed to it, but not necessarily capable of readily killing, well….
What Thanos does with the Time Stone once he gets it isn’t much less impressive than what Doctor Strange uses it for:
He uses it to reverse time, restoring Vision and the Mind Stone itself from destruction. Roughly similar to what Strange did with Dormammu.
Intangibility is not the main power of the Mind Stone. That’s mind control, as in what Loki did.
that was the last stone Thanos obtained,
it’s not shocking that we don’t see much of its powers.
The Aether could undo the universe, but it’s important to distinguish between what a stone does by itself, and what its wielder is capable of. See below. Further, Thanos doesn’t destroy the universe, but, well….
In fact, Thanos uses the Reality Stone for various things that Malekith seemed incapable of. Arguably, he actually possesses a better understanding of it.
We don’t really know what the Soul Stone does in the MCU yet.
Energy creation is very much a side effect of the Space Stone. Its main power is to warp space, creating portals.
Thanos does a lot of this in Infinity War
Finally, once Thanos obtains all the Infinity Stones, he does something that no Stone individually, whether in the hands of a sentient being or not, has been able to do:
He kills half of all sentient beings everywhere in the universe.
That said, it’s clear that the capabilities of the Stones are limited by their wielders. No one can wield their full powers. Most creatures need special items (e.g. the Infinity Gauntlet) to even try. Thanos may be very strong, but we don’t know how easy it is for him to wield even one, let alone several simultaneously.
Also, he’s not one for cleverness and planning (in the MCU, anyway). Yes, he was able to manipulate the events so the Infinity Stones came to light, so he’s not unintelligent, but he doesn’t seem to like this much. He seems to prefer brute force. So he doesn’t seem to bother being very clever with the Stones. Once he has one, he just smashes through toward his goal.