Why do my 6S 7" quads shake all the time?

I found this article about drones, and here it is.

Firstly, here’s a video showing what I’m talking about.

6S 7 inch quad on BF 4.1 shakes horribly.. all the time. (1 min 17 sec)

It feels like like flying in propwash 100% of the time, and everything I’ve tried to fix it only makes the problem worse.
It’s so bad that with my DJI HD gear on there, it’s almost nauseating. I’m about one more session
away from throwing this thing in the trash.

Frame and gear
I’ve had the same problem with two different 7 inch frames with various different gear on em.
First was a GEPRC Crocodile 7, and the latest is an open source FR7.
Croc7 started with GEPRC’s own dual gyro F7 FC and 4in1 ESC, then I switched it over to Kakute F7 HDC stack when I added the DJI HD gear,
and then moved that same stack over to the new FR7.
Both frames, both FCs.. shake the same.
Former had 2306-1700kv motors that pulled it around the sky just fine, but saw the same shaking.
Some people say 7 inchers need big motors, so I put giant Xing 2806.5-1800kv motors on the FR7.
The Xing motors run smooth as silk and with fresh props, there are no vibes in the frame at all,
at least when only one motor at a time is running.
But it shakes with both small and big motors. If anything it shakes even more with the bigger, more powerful motors.
Croc 7 frame was a bit twisty. FR7 frame is super rigid. Both shake the same.

Tuning
I started with BF4.1.1 with rpm filtering enabled. Am now running BF 4.1.6 with fpm filtering enabled.

Starting from bone stock PIDs.. It’s nice and smooth in a hover, but starts shaking at any throttle position above a hover.
Gets really bad when I give it a quick punch of throttle. Stays consistently bad if I just throttle up to go fast.
Downright horrible in the wind.
People say 7"ers need higher PIDS so I raised the PIDs and it got worse.
Lowered the PIDs below stock, and it got no better than stock.
I saw someone else running very low I-terms on their 7incher, so I tried that. Improved ever so slightly,
but still generally shakes all the time, and just as a bad when I throttle up.
Tried disabling ANTIGRAVITY because that raises I-term with throttle. No real improvement.

I’ve been through all the suggestions in both BF 4.0 and 4.1 tuning tips with regard to larger frames (changing the
filtering frequency ranges and such), and it produces no improvement, or got worse. The onboard
audio indicates all the motors are just warbling all the time when I’m flying at speed.
The vid above is from DJI onboard DVR. Gopro audio makes the warbling even more pronounced.

I’ve been flying multirotors for a long time, so I’m generally familiar with what
P too high looks/feels like, most easily seen with extra ringing after sharp inputs, but
I get none of that. Sharp inputs produce razor sharp starts and stops.
D term too high usually produces much higher frequency oscillations and hot motors if there are
problems with filtering, but I don’t see that.
High I term (usually hard to make it too high) causes low speed oscillations which are
generally slower than what I’m seeing here and not randomly on all axes like this shaking is.

Every change I’ve made to FC, frame, motors, props, and tuning has either resulted in
no improvement, or made it worse (sometimes much worse).

What I’m looking for is someone who has had or recognizes this specific problem, and knows how to solve it
as I’m really tired of just trying random suggestions. I’ve been fighting this for varying degrees for months
and switching to HD video gear, has just highlighted how bad it’s always been.

FWIW, I’ve got a 5inch quad using the same Kakute F7 HDC FC that flies like it’s on rails, even in very windy conditions.

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