Dave Filoni states the following:
''Maul (Rebels) hasn't underwent any personal growth besides in his anger and need for revenge''
1:38:55 to 1:39:07
From the above quote, it is apparent that he is treating end of CW Maul and Rebels as the same. So at the very least, no indication of Rebels Maul having declined in any way (contrast this to the way he talks about Season 4 Maul vs TPM). However, he specifically states that Rebels Maul is angrier than TCW Maul and has a strong desire for revenge.
We know that anger fuels darksiders, especially Maul. Given that it is stated in Skywalker Saga that anger fueled Maul's powers:
''Qui Gon Jinn was strong, but the Zabrak seemed stronger, full of
Force users typically grow more powerful in the force with age, as stated in Rise of Skywalker novelization:
Further re-affirmed with ESB Yoda furthering his connection to the force. The only counter-examples are FUs who intentionally neglect their connection to the force. Such as Obi Wan in his TV series, and even when he regains his connection, he strains to lift Leia. And it takes him several days of reflection, fighting and mental resolve to return to his ROTS level force power.
At best, Rebels Maul might be physically lesser than his TCW self at base, but he should be able to augment himself to and beyond peak TCW levles.