I have only 170 followers and 2 mail subscribers at Medium and 20 mail subscribers at substack. And yet every free post i post at substack goes to all 20 subscribers and all of them seem to read it.

Also you are dead wrong on your 'stick to one platform' advice. Platforms can change algorithms or rules overnight. They are private companies. If you dont have a mailing list of your own that links you to your connections, you would go down with the platform's algorithm change...