This is a terrible idea, and borderline irresponsible. One of the key reasons that Lemmy doesn't subscribe by default is to avoid forcing servers with many communities to waste time/CPU delivering messages to servers where no one will read those messages. By subscribing to everything, you're telling all those overloaded servers to waste time sending content to your server that you'll never even see.
- It also will massively inflate your db by multiple GB/day.
- It will maximize the chances of you downloading and hosting copyright infringing content and content that may be illegal in your jurisdiction but not in the jurisdiction where it's hosted (loli, etc).
It is much MUCH better to just hit lemmyverse.net and subscribe to 10-100 communities you care about. If script accepted a list of community-urls and automated subscribing to those, that would be super nice. Subscribing to the entire lemmyverse is terrible for your server, for your hosting liability, and for the lemmyverse's performance.