Wanted to add to this one

Use that email list, upload it to Facebook, and run targeted ads just to your email subscribers

Facebook will also allow you to create lookalike lists of folks unknown to you. I've heard some great things about the responsiveness to these lookalike lists.

The giveaway is a powerful tool, one that I used extensively to build a list when I was self publishing. I would caution any doing this however to not go extreme in building the list. By that I mean not only should you ensure everyone opts in and has an opt out button, you should regularly cull your list as well. You will get a percentage who signed up just to get the freebie and they will never open anything you send them after that. Not only do email campaigns cost per the size of ones list, one can actually get penalized if the unopened emails grow to large. As well as some folks will forget why you are emailing them after awhile if they aren't interested and could begin marking your emails as spam instead of opting out.

Invest in a long term SEO program. Getting your website to the top of Google has significant value and can scale your business up to a whole new level!

I would also caution folks about spending to much time on Google placement. You obviously want to do as well as possible, which is why you should consider Golden Keyword ratio. However, Google is the first to admit that over 90% of all websites will never get even one visitor from them.

https://ahrefs.com/blog/search-traffic-study/

Knowing this, one should focus their efforts primarily on creating funnels to ones web presence. Unfortunately the average person will never get the rep with Google that many of the corporations you've worked with will get as far as being considered an authority.

Some alternatives could be self publishing as I was doing, but tainted more to get folks to your upsell on your own website. Using other folks email list. While this can be costly, over time one will begin possessing their own quality list. The list is like having your own printing press for money if built and used correctly. By correctly I mean not overusing it, and making sure most of the email is not trying to sell something, but being helpful to the target audience. This makes them receptive on the occasions you do pitch something. Receptive because studies have shown you need to interact with a potential customer multiple times before they actually buy something. Best to make those interactions focused on them to build your rep as someone who cares about more than the quick buck they represent.

Hope you don't mind my adding my observations. Great list and welcome to Blurt. You should change one of your tags to hello Blurt since this is your introduction post.