So I had my 7" quad tuned perfectly on iNav, set it down for about a month, and now I can't seem to increase the throttle very far without a very clear shaking and oscillating noise starting. The shakes stop as soon as I bring it near hover again. As far as I can tell, the whole unit is pristine aside from a tiny chip on one of the props. Any ideas what could cause such a huge change? Can provide settings, blackbox, and video if needed.

i would check all screws and frame rigidity by hand

Then, remove the props,, hold the quad down.. in betaflight being each motor up (one at a time) and see if u feel any difference..between the motors

All screws are where they belong, the frame is good and solid, and the motors happily spin all the way to 100% without any sign of distress.

Re install the props..make sure they are tight.. NOT just the nuts… also.. see if ur fc is firmly mounted

Oh yeah, these props aren't going anywhere. They attach to the shafts so firmly that it would probably fly without the nuts. The flight controller is exactly where I installed it. No slop in the attachment.

Just to eliminate the obvious, have you tried, or do you have a new set of props to test?

A slightly bent propeller will cause more shakes at higher rpm.

If you don't have a new set of props you can just spin them one at a time by hand, and see if the ends of the props are the same distance from the table, or from a specific point on the frame.

^ I do this check fairly often.

The props are still super straight, with just a tiny chip on one blade. That chip was present on previous smooth flights. One thing I've been wondering is if temperature changes can invalidate a good PID tune. My previous flights were in the range of 70-80F, but my current ones are around 30-40F.

Update!

I dialed the D gain down a fair bit and the problem has ceased.
Thank you for your attempts to help me, everyone!

Sorry, I have no skill/experience in this area, but.....

Why did you have this problem in the first place.  You changed a setting to fix it, but you didn't change a setting to cause it.

Am I missing something?

Is there a setting drift occurring in INAV? Because by my logic, if it flew fine a month ago, you changed no settings and it didn't fly fine a month later, something has changed.

I want to say i need an answer, but maybe the universe (or matrix) is just having a hiccup.

The only thing that's changed since I had it working is the temperatures outside. Outside of that, I've not touched a thing. If you don't feel like blaming the weather, you could blame one of my cats. He's probably decided to off me in one of those fabled quadcopter beheadings.

Update: From the info I've found, the humidity and temperature outside could very easily have shifted the air density by 5-10%. If my PID tune was right up to the limit, I'd say it's quite possible that it's the cause of my issue here. My tune was rather aggressive due to the quad being rigged for efficiency, so getting things solid had me using values that would be suicide on typical builds. Will just have to remember that I have to loosen this one up when it gets cold out.