No power was a tree without roots or water without a source. After the Ten-Tails revived, the first thing it did was unleash Deep Forest Emergence, using Infinite Tsukuyomi to harvest the Shinobi World's chakra for itself. That was why it became so powerful.
Though humanity's devout faith could rapidly nurture Dreamplay, which possessed the properties of a fragment of the Demonic Statue of the Outer Path, time was simply too short. The imminent Fourth Shinobi World War left Mizuki with no time to accumulate strength.
Dreamplay's ability to absorb chakra had already far surpassed that half-dead fragment of the Demonic Statue of the Outer Path from before.
To rapidly increase his strength, he could use special methods to absorb natural energy without restraint.
But Mizuki did not even dare imagine the consequences. Even the God Tree had needed countless years to grow before it dared harvest natural energy and chakra without restraint. Yet it had still been sealed by the Sage of Six Paths after his sudden emergence. When it revived again, Sasuke Uchiha and Naruto Uzumaki joined forces to deal with it.
If Mizuki dared do that, he would either be devoured by natural energy or, like an immortal undergoing tribulation, be struck dead by the heavenly lightning tribulation... or some other calamity, leaving not even ashes behind.
Trying to openly plunder the Shinobi World's natural energy without the constitution to withstand it was simply courting death.
As for the remaining methods, he could always rob the rich and redistribute the wealth. The fattest target was naturally the Demonic Statue of the Outer Path in Uchiha Obito's possession, with the Tailed Beasts sealed inside it. Unfortunately, Mizuki was not yet a match for him.
The only other targets were the Tailed Beasts that Akatsuki had not yet captured.