Everything is in the title 
My thanks to Achievable for the help!! For what it’s worth, I tried to summarize my experience below:
I had purchased an exam prep book from another vendor but was not going anywhere; I found it extremely hard to understand, the outline of the textbook was built after the exam (chapter 1: knowledge of capital markets, etc.), with FINRA rules sprinkled here and there, vocabulary not explained, random bits of information not connected to each other, etc. In addition, the practice tests seemed poorly calibrated for the SIE exam, in terms of level of detail and topics tested.
With 12 days to go, I went for Achievable, following the advice found in an Amazon review of the above. It made a huge difference! I found the Achievable textbook to be much easier to understand and digest, in terms of structure (start with the basics - common stocks, etc. end with the regulations/requirements for finance professionals), and content, i. e. how well things are explained. In hindsight, the practice exams very well calibrated for the actual thing. I had some hesitations because I found discrepancies between Achievable and other practice tests elsewhere, but that got resolved with prompt feedback from the team (see SIE - topics not covered in textbook (?) - #8 by Kenny_Green). Hopefully this will give some confidence to others who experience the same!
Some more context:
-I had some education in and practical knowledge of corporate finance / M&A, although not in public markets. Also, I do not live in the US, so I had no real-life experience with education or retirement plans, etc. I probably had ways to go.
-This took focused work, having a day job and family, but I did not mind the crunch. I put in 4+ hours every day, excluding week ends (things to do!)
-I really liked the custom-generated plan, it helped with stay on track, and even motivated me to get ahead. Again, I found the textbook very well structured and easy to understand. The quizzes at the end of each section are great to memorize and test your actual understanding of the topic. I personally like to take written notes when studying, it helps me memorize and put my own emphasis where needed.
-I took 14 full practice exams in total, starting at the end of day 2; I knew I would not score well on items I had not studied yet, but this made for some “aha” moments when I got to them later. By the time I finished the text book I was scoring consistently in the 90s. Doing too many of these can be a pitfall, as you tend to memorize Achievable questions instead of getting ready to answer the FINRA ones
-on the day of the exam: last look at my note book (beginning to end), at specific sections of the textbook (more obscure topics like retirement plans), took another couple of practice exams, and a double espresso
-during the exam: time allotted is more than sufficient (I used about 1h in total), so my advice would be to carefully read every question to adapt to their specific wording, detect the subtle negatives and little traps.
Thanks again, I think I will trust Achievable once more for Series 63 but will have to go somewhere else for Series 79, unless it will launch here soon…