Hi and welcome to GMAT Club!
There are definitely several elements to success on the GMAT. One aspect is the material and the other is the energy to cover it. Those two lines intersect somewhere and you don't want to have too little material since you will be under-prepared or too much since you will burn out. I definitely understand your point about the amount of time it may take to get to a certain level. But....
Here is the but - how much improvement are you looking to gain? This is the key question - how much do you need to improve by? I would say that most people who are effective students who review their yesterday's work every day and refresh their reviewed material weekly can probably get 150 points. A person with decent dedication but slightly distracted can get 100 and those who are distracted probably 50. On the other hand, some serious changes to one's routine and learning habits could result into 200 points. Going above 200 points is Top 1% type of thing.
It is possible that you are starting out at 540 level and setting your sights on 740, you are telling the system - "Give me 200 points" and it does just that. The system is a system and it does not lie but rather has a sense of how much effort requires to improve from each of the levels/points. 200 points of improvement is a BIG effort. I improved about 200 points (210 if anyone is counting) and I had to rearrange my days, do nothing else for 3 months (though I don't recommend it) and focus on the GMAT and chasing every mistake. (by the way, you said something that is a bit concerning - you are starting to judge material and what will show up on your test and what will not. Here is the thing - You don't know. You can't say that, not if you are looking for a 97th percentile. You set your sights to beat everyone - beat 97% of the applicants and you will have to solve each and every question that comes to you. If you are looking for a 670, which is around 80th percentile then you can be picky but if you are looking to be top 1-3%, you can't say that. You must treat every question as though it will show up on your test because once you let one question by, it is a slippery slope. You don't realize it but you start telling yourself inyour mind not to worry about some questions and then others and so on. Soon, you have a whole book of questions you should not worry about). Anyway, I am sure you are not thrilled to hear about it, but it is better to hear and decide now rather than after 3-6 months down the road.... by the way, don't spend more than 4 months studying. 6 Max, so shoot for 4, and you will have some reserve time.
Anyway, speaking about books on the other hand are fairly straightforward. They are not dynamic as a course and whether you are looking for 50 or 200 points of improvement, it is the same set.
Whatever the material is, it is important to identify if you are getting the improvement you need. I have devised a very crude system that all experts usually hate because it is imprecise and probably gets misused but the way it works is based on 10 questions of easy, medium and hard level (3, 4, 3). If you get 9 correct, you are at a 750 level. If you get 8, then 700, and if you get 7, then 650, and if 6, then 600, and so on. I would test yourself after each chapter (every day basically) and track your performance and if you are matching the goals you have set for yourself. If you think you can get to the same score with half the time, that is awesome but something tells me that a dynamic course (and all the brainpower of people behind it) may know better than I if I just started out with the GMAT so I would not trust myself. I think at the end this is similar to building strength. You will have to put the effort in. However, if you like certain exercises better, it may be less tedious and more enjoyable and thus more likely to succeed so you want to pick something that motivates you to continue since it will be a grueling hike. Also, some people better respond to studying with the book (which is probably a smaller number in these digital days but there are advantages books have such as ability to turn off electronics and study undisturbed; i would not do it in a park or a bus however - that's not productive).
P.S.
MGMAT has the Math and Verbal Foundation books in addition to the main series for those who may need an extra refresher of how to do long division, fraction operations, and all those good things we hardly use these days. You don't need them if you are starting out at 540 or 600 but definitely yes if you are at 450. Which of course elongates the prep time back to the summer timeline.
P.P.S. Technically now you have 2 sets of materials - that's good and bad. That's like cooking 2 meals or riding 2 bicycles. The meals won't cook faster and you won't get there faster... so you have to avoid falling into the trap of "incomplete fallacy" You are assuming that doing
TTP +
MGMAT is better than
MGMAT or
TTP on their own and that's not correct. What will end up happening is that you will cover the same material twice and you will miss the important parts you needed to learn most. Also, you will always feel that you have an incomplete set of materials since you skipped this or that. It is better to have just 1 set than try to cover 2.