I wonder if you string together enough words can it be a valid key?

I would hope so, sentences and words are some of the most secure passwords/phrases you can use

Words are the least secure way to generate a password of a given length because you are limiting your character set to 26, and character N gives you information about the character at position N+1

The most secure way to generate a password is to uniformly pick bytes from the entire character set using a suitable form of entropy

Perhaps, but part of having a password is being capable of remembering it. I understand there are solutions to this, but I personally prefer to be able to recall my passwords manually (is that the correct term?). I don’t like the idea of needing a second device or piece of software to store my pw that’s a nonsense string of random characters.

I know they’re not as secure, but I like to use passphrases.

we are talking about RSA keys - you don’t memorize your RSA keys

if you rely on memorizing all your passwords, I assume that means you have ample password reuse, which is a million times worse than using a different less-secure password on every site