That jittery, hard to keep in the same lane problem is often caused by something that is causing a very stiff ride.

Getting to recommended tire pressure which is on your door jam and also on the tire is the first thing to do. Unless the tires you are using are not recommended for your vehicle they are unlikely the cause. If the tires are under or oversized, sidewall to hard or too soft they will be a problem but if there are in spec tire to tire will feel different but should not be the culprit.

Alignment can also cause the problem. There are good and bad alignments. I would have it checked to make sure it is within BMW specs or has gone out of alignment.

If it has gotten worse over time and your suspension has 75,000 miles or more of wear there are several components that could cause your problem. I would have it thoroughly checked over even if it is not toward the end of normal life cycle.

Generally the problem is magnified the faster you go. If it is in the steering the vehicle won't feel jittery but will wander at slow speeds. Steering problems hit my critical to fix ASAP list.

Some roads will cause that feeling when all is OK. In your case if you are more than 5 pounds over inflated I think it is at least contributing to the problem by losing some of the cushioning the tires do and perhaps losing some of the tire patch.

It is indeed 2.2 - 2.7 bar shown on the side of the door, depending on the load. However, the tires I've got on are of no known brand and pretty useless...3.6 is the lowest I can have in them if I want to avoid a "boat" feeling. Based on your info I'm sure the issue can be reduced heavily if the tires and suspension parts are upgraded/changed. It's on my loooooong to do list.

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Increasing negative camber may help, however, negative camber will increase tire wear. If you go to the max of the alignment spec for negative camber the wear shouldn't be extreme though it will vary by tire hardness.

Over inflating the tires will result in less tire contact patch with the payment. Less contact patch will result in less than optimum handling. Fill the tire with the correct air pressure as stated on the door jamb for the car.

I just replaced the tension strut bushings and ball joints and it has dramatically reduced the problem. I think I'll also replace the lower control arms because the previous owner replaced only one side and I think the other is worn now. Best to replace both. Also my tires are worn a little uneven, so I'm sure this is contributing to the issue. I am running the OEM tire size and pressure is around 38 PSI which is the recommended pressure for these tires per the manufacturer for my X5. I'll update after the control arm replacement.