I actually have a pretty good grasp on MMT and have written an article about it which you can find on my page. MMT is not “widely dismissed” by economists.
Printing money is not just another form of taxation. I don’t even know what that’s supposed to mean. Nobody is claiming ‘free lunch’. But there are more or less efficient ways to organize resources, just as 5x5=25 but 8x2=16 even though they both sum to 10.
What is ‘fixed’ are the real resources that exist in the world. Financial wealth is not fixed, and crediting some bank accounts does not decrease the value of other bank accounts to an equal degree, whether by inflation or taxation.