First off, IF does not have auto pilot, and it does not have trim tabs.
In IF, you control your pitch with the elevators. The auto VS locks your elevators in a selected position, so it imitates a trimmed aircraft flown with a joystick or yoke instead of a gyro, except with IF, your are locked in into that trim position until you either manually adjust or disengage the VS.
The reason we experience the nose bump is because when the auto VS is engaged the elevators very quickly reset to the zero position, before returning to the position you selected.
To avoid this you need to “trim” your elevators to the same position they will be in at a preselected speed, weight and climb rate. This “trim” position will now be the new zero position.
It involves some trial and error, but made easier if you use the same constants.
For example ;
Make 230kts and 10 degree climb as your constant. Select a take off weight and use the same weight each time.
Now it is a matter of adjusting your “trim” until the bounce is eliminated.
It is actually not as difficult as it sounds.
Here is a take-off portion of a flight video I took today.