On covers, it pays to comparison shop. You can find them in a wide variety of price ranges, though it looks as if you're using the same basic image each time, so it may be a while before you need to worry about that.
Most designers display their previous work, so you can get an idea of whose style might be most consistent with your desire. Some also have a certain number of premades (cheaper), but a crafted-to-order one gives you a lot more flexibility.
Professional editing is also very important. You want your book to be in its best possible shape before publishing it. You may need to shop around for an editor with the right background and price range. As with cover designers, some advertise here.
Timothy, what's the current thinking on release pattern? I'm thinking some kind of rapid release with that many books already in the pipeline, but I know conditions have changed.
Anyway, once the book is out, exposure is critical. Experiment with social media to see what kind of results you get. (But be aware that most social media has moved more and more toward pay-to-play. You used to be able to get organic engagement. Now, it's hard to make any progress without an advertising budget.)
Book Funnel does have a monthly fee (check plans for details). But for that fee, you can enter as many sales and mailing list promotions as you want. Sales promotions on Book Funnel will typically net you some sales. They are most successful with reduced price books. Last time I checked, I got a 2-5% conversion (clicks to sales) on full-price books and 5% to 25% on sale-priced books. Choosing an appropriately themed giveaway is critical. At the very least, you want a genre match. But the tighter the theme, the higher the conversion rate. I once entered all three books in a dragon-related trilogy into a dragon-themed and got over a 50% conversion rate. Also, promotions with more authors involved tend to get more eyeballs than promotions with fewer. There are also more competing books in the bigger ones, but on balance, I think they still tend to be better.
Mailing list promotions involve giving away a freebie in hopes that people who like the looks of it will subscribe to your mailing list. (And yes, ideally, you should have a mailing list.) Something like a short story related in some way to your book series would be good, as people who like it may go on to read the books. Some promos will allow for novel excerpts; others will not. Always read the promo description to be sure.
For a new writer, Substack is a good deal. Why, you ask? Because it gives you what amounts to a free website, a free mailing list (most providers will charge over a certain size), and a large potential audience, much more so than you might get on your own blog. (Book Funnel doesn't allow Substack integration--yet--but it does let you collect emails on a spreadsheet and import them.
It also creates another potential revenue stream, though this is a slow process. Substack runs on a subscription model. You can make content free, you can make it available only to free subscribers and up, you can make it available only to paid subscribers. You can decide separately for every single post. You control whether you charge and how much you charge. You can give discounts for yearly (as opposed to monthly) subscriptions. You can run periodic sales. You can create an even higher founding tier or not.
What do you do for content? Some people post about their writing journey (but frankly, that works better if you already have fans.) Some people post teasers (surprisingly popular). Some people post short stories related to their novels. In either case, it's considered appropriate to link to the novels you are trying to draw people to in each post. (I do get sales that way.)
A lot of people also do serial fiction on Substack. Those can be anywhere from just a few episodes to their whole novels. I try to create serials related to my novels, and I've also started publishing my WIP that way (which gets taken off Substack, except for a teaser, once the novel is published). How much you do depends on how much time you have. There is a thread in which forum members link to their substacks.
https://writersanctum.com/index.php?topic=6021.0 Look around, browse, see what other people are doing and whether or not you want to join.
Oh, and you can also publish related material like book and film reviews in your genre. Many people do that.
Substack also has Notes, a social media platform where you can advertise your posts to nonsubscribers. There are other discussions as well. Introducing yourself gets a lot of attention. Stay well clear of political discussions, but generally, the atmosphere is supportive.
For further infomation, see
https://writersanctum.com/index.php?board=69.0