Yes, your use of the B12 for months before testing would have raised you to "new" levels.

On the one hand --being over 1000 after taking the high dose supplement shows it is being absorbed properly and working for you.

On the other hand you don't know where you started from.

I would think your last test reflects the daily use of high B12 doses.

It can take time to come back down. Perhaps a long time. Part of the B12 you took, would have been stored in the liver for a "rainy day".... So if you stop now, it could be months or a year or more before you get really low, if your liver stored what it should.
This is a survival feature for B12, since it comes from animal protein, and early humans encountered times of famine and loss of animal prey, or the ice age, etc.

I think you will find Kaiser not conducive to much testing.
They might screen you for hemochromatosis, which can cause
PN, and other nasty consequences, because they did a study several years ago on all their members and found many hidden patients with this problem. (including some of their physicians!).

But their doctors are bound by internal rules on what drugs to use and what tests should be ordered. You may run into that wall when you use that insurance.