Although it is true that using fever mode gives better visibility (and that, perhaps, makes it a little bit easier to hit the notes right in several instances) I still tend to avoid it because it is hard to tell if you missed something or not while you are in the fever mode. Without fever mode you know immediately if you missed anything because the fever bar goes back to the minimum, two seconds after the missed note. If fever mode slowed things down, even by 5%, I would be using it nonstop, but it doesn't change the speed of the notes, it just makes them a little easier to notice AND it also removes the ability to see if you missed or not on the spot, which is a problem. Basically, the fever mode is only great for scoring more points in a portion where you know that you will miss [it will keep the fever bar high, even if you miss] but it doesn't help much for the SSS ranks.
For the hardest 5-second portion in hard mode Senbonzakura (basic stage 06), I think I have an idea of what would work to guarantee passing it, but, you know, this is the sort of thing that makes you realize that it is ridiculous (and probably not worth it, if this is what has to be done to pass it). The idea is to take a video of the level without the wands hitting any note (to see all the notes on the screen clearly, without your hands in the way); then edit it into a 6-second video of that "impossible portion" only (using Sharefactory on Ps4); then use a usb and take that video to VLC Media Player in your computer and set the playback speed to 25%; then see exactly what the hell is going on during that portion and memorize the easiest way for you to do it without missing; then raise the speed to 35% and do it again (to get your eyes used to the faster speed, otherwise you won't get it at 100%, even if you can get it at 25%); then redo that for +10% speed increments until you are fully capable of noticing the entire pattern and knowing that your hand-eye coordination will be able to do it at 100% speed. This is what people do with the practice mode (that has lower speeds) in Beat Saber's expert+ levels and in Audica, but because there is no practice mode in Hatsune Miku VR, you have to do this sort of thing instead for the hardest part of the hardest song.
For the record, I ended up at #76 in the leaderboard for that song without the SSS, which means that probably less than 60 people got the platinum. It's pretty much the same story as in Audica, but with just 20 stages to complete (10 songs with 2 modes each), and here you need full combos (no misses) instead of "SS rank" scores. It takes far less time to complete for sure, but it is as hard as Audica. I will take a break from it and come back in a few days to use the "slower video" method I mentioned above. If that doesn't work, I got plenty of more fun stuff to do than grinding Hatsune Miku the game lol 😁