A reader magnet can be MANY things!
I didn't have one for forever, but still got organic sign ups. In part, because I spent a lot of time designing and optimizing the content of my newsletter. (Content relevant to the interests of your genre fans= a newsletter they look forward to getting.) But when I did finally produce a magnet, I saw a huge jump.
My magnets have grown over time. Just start with what you have.
Got a deleted scene? Or character sheets? Offer that.
Then, if you get time to write something else-- a short story, a bonus epilogue, etc. something bigger and better, then offer that.
Then if you get the time to write a prequel novella or a bonus novel? Then offer that.
You don't have to have it all figured out right this second. Your reader magnet(s) can grow and change over time.
As for diverting time away from "paid" writing...
No writing you do is ever wasted, so that short story or novella you offer to your list today? You can always repackage it and sell it later down the line.
(ie I'm writing short stories in my series universe. It started as me just trying to learn HOW to write short stories. I release them to my list for free as a bonus for being on the list, then I dangle them as sign up bonuses. And, once I have enough, I will repackage and sell a short story anthology as a full price ebook/paperback.
I "advertise" my freebies in the back of my books, on social media, and on my webpage -- freebies are here:
https://dmguay.wordpress.com/freebies/ Two short stories and a free 45k word ebook guide to B movies. I write horror comedy, so this is relevant to the interests of my fans-- and they can get them either direct download link via bookfunnel AND I have links to all of them in my newsletter welcome email. )
I found the bonus content episode of Six Figure Authors SUPER helpful. Lots of ideas
https://6figureauthors.com/podcast/how-to-use-bonus-material-to-sell-more-books/Oh, and as for $$$. When I wrote a 12k word Kevin vs The Mothman, so many people asked for it in paperback. I hesitated, but eventually released it in paperback. It's sold over 100 copies since August in paperback, which basically paid for the cover. Having a paperback hasn't reduced its effectiveness as a sign up bonus, either. Not sure why. Maybe some folks will buy everything you do, but just aren't interested in another email? Who can say.