I don’t know if I can be of much help (don’t know enough about your situation), but I thought, nonetheless, I would write some words on how I usually tackle the test.
I think the most important part for N2 and N1 is actually engaging a lot with the language. You should be reading books, manga, articles, watching YouTube, dramas, Anime, listening to audiobooks, play japanese games, write online with Japanese people, talk with people etc. as much as possible.
All that should happen anyway since you want to engage with the language regardless of if you are currently planning to take the test.
I have some advantage since I feel like my foundation is pretty solid. Around the middle N4 level, I did a working holiday in Japan and lived in the country for nearly a year. During that time I engaged with the language a lot and on top did some intensive study on my own. That catapulted me to a solid N3 level in a pretty short time, and all the basics got solidified, which allowed me to build on slowly from then on.
I took the last N2 a few years back and since then have only lightly engaged with the language until I felt mentally ready to tackle actual preparation for the N1 this December. Rushing from one N level to the next and only working with textbooks never felt quite right to me.
When I feel ready to tackle the next N level, I usually start to learn for the test roughly 6-4 months before. I replace some of my usual engagement with the language with actual textbook study + intensify my engagement hours, but I make sure to keep some fun parts in my regimen as well. I would say actual study is roughly 1 hour a day? Less during the week, a little more on weekends.
The resources I, personally, use:
- Try! Series as a starter to get an overview for the level
- 新完全マスター文法 to focus on finer grammar nuances
- 新にほんご５００問 added in after I finished Try! slowly doing the 3 rounds through it.
- 日本語の森 preparation videos for the level I want to take + one level down (usually start roughly 1 month before the test), refreshing grammar + vocab + also decent listening practice.
- 1-2 mock tests roughly 1 week before the test
Depending on where I feel my weak points are, I have other materials to support.
Usually reading is easy since I already engage with Japanese so much → no extra prep. Listening can benefit from some dedicated listening exercise drills since the questions and conversations can feel pretty stilted compared to normal Japanese content → maybe some minor extra prep (新完全聴解). Kanji: I already finished WaniKani before I took N2 and because I’m reading a lot, I usually know way more (readings and Kanji) than the JLPT even requires. I honestly don’t get people who feel Kanji is the hard part 
So my biggest gap is mostly vocabulary. I haven’t found a shortcut for that yet. That is another reason why I keep a lot of time between levels. I mine from the material I engage with and do roughly 5 new vocab words each day (even when not preparing for the test). I also have some dedicated vocab books, but I just can’t seem to get more than ~5 Words per day in my head 
. The amount of vocabulary you need to know for the upper levels is actually decently big, + a lot of pretty similar words that have dedicated usages start to appear.
Since I started really early with N1 preparation this time around, N2 was a good while ago, and the N1 test is half N2 topics anyway, I started with a short N2 refresher by quickly going through the old level N2 as well. A solid foundation makes advancing so much easier.
Overall taking the test is pretty easy for me since I do well in fixed test settings and feel well prepared when taking it. Both of the last 2 tests were close to full marks, so the system is working for me. As a downside, it is definitely not optimized for speed, and I wouldn’t recommend it to anybody who has to get the test ASAP. On the plus side, I actually enjoy engaging with the language and I don’t have to do 3-5 hours of dedicated study each day, for which I wouldn’t even have time.
Not to discourage you, just to put that into perspective, that’s a maximum of 480 h of Japanese engagement. Given people living in Japan and going to a Japanese language school need roughly 900 active study hours, even if you did that 2 times already (for your 2 fails) would be the minimum work I’d expect for a pass. And only if you actually engage with the language in other ways next to your studies as well!
It just takes a looot of work and a decent chunk of time to actually get good at Japanese if you don’t have a head start like Korean or Chinese.