There are many prep programs out there, ranging from free to pretty expensive. No matter what you do, you need to treat your ACT/SAT prep like a final for a class you are "decent" in, but not an A+ student.
If you have tried the free material and you don't see the results you want, it may not be a matter of the "free" content from Khan, CollegeBoard and ACT not being "good" enough. It may be an issue with how you study, the environment you study in and whether or not you have trouble with standardized testing or a specific area of content (or subcategory of content) in general. Your score reports and many prep courses have an option of breaking down the subcategories of question types within a category and that is always a good place to start to see what you are struggling with.
The next level up from free is buying books, "on your own courses" or all online programs (like ACT online). These can be cost-effective options. From there you have many companies like Kaplan, Princeton Review, etc. doing in person (no longer) and online "group" tutoring. These can be costly but they can be a good option depending on the quality of the instructor and the group size. Many of those have a "score guarantee" starting with a baseline score you provide from an official test or one of theirs. I think you need to check out the "guarantee" though, as it many times may just be another course for free if you met all the criteria in the first place (didn't miss a "class," turned in all the homework, etc.) From there you have several companies (and many individuals) that provide private tutoring from experienced and "professional" tutors - the most expensive option of all. If you have the resources to do it, it might be the best option to really drill down on where you need help, but again it could be very expensive.
What it really comes down to is that you need to identify your weaknesses, the best learning methods that work for you, your level of determination, how much time you have to put into it and the financial resources that you have available to expend on the ACT/SAT prep.
Two major considerations now: (1) most would-be test takers are out of school, and there will be extra time to study - but the "in home" studying environment may be a challenge for some right now; and (2) with many of the spring tests being cancelled or moved, there will likely be less test opportunities going forward this summer and fall so it is critical to get it right with less chances to turn in your best superscores. It is possible that the ACT/SAT people add more tests as well, but you just never know.
Good luck and stay the course!