The spell immovable object from Explorer's Guide to Wildemount (pg. 187) says:
You touch an object that weighs no more than 10 pounds and cause it to become magically fixed in place. You and the creatures you designate when you cast this spell can move the object normally. [...]
[...] a creature can use an action to make a Strength check against your spell save DC. On a success, the creature can move the object up to 10 feet.
At Higher Levels. If you cast this spell using a spell slot of 4th or 5th level, the DC to move the object increases by 5, it can carry up to 8,000 pounds of weight, and the duration increases to 24 hours. If you cast this spell using a spell slot of 6th level or higher, the DC to move the object increases by 10, it can carry up to 20,000 pounds of weight, and the effect is permanent until dispelled.
The two phrases I am interested in here are:
You ... can move the object normally
it can carry up to 20,000 pounds of weight.
Suppose I cast immovable object at 6th level on a thin sheet of plywood. I then proceed to stack 19,999 pounds of gold ingots on top of the sheet of plywood. I then attempt to move the sheet of plywood normally.
Can an object under the effect of a 6th level immovable object spell still be moved normally while it is carrying up to 20,000 pounds?
Note, 20,000 pounds of gold is not very big in terms of volume. The density of gold is 1206 lb/ft3, so 20,000 lbs of gold would only be about 16 ft3, or a cube about 2.55 ft on each side.