Btw, She's a Beast, one of the newsletters I subscribe to, uses Ghost https://ghost.org/ as a substack equivalent, and there's a huge disclaimer here that I do not know what their policy around platforming Nazis is, but it seems like if you're using Substack it might be worth looking into and finding out! I think there's a FOSS version but also a hosted one because hosting is a challenge.
(and lmk, I'll edit this if they turn out to be terrible too!)