Pawn movement is part of “simulation” not part of “animation.”
What you want here is simulated inertia.
Neither timelines, nor enhanced input, will give you that. You have to instead build the notion of inertia, and acceleration, into your control and simulation scheme.
If you’re using Unreal physics objects, then do this with friction, and forces. When input forward is received, add a force to the simulated tank object, in its forward direction. When not received, let friction do the work of slowing it down, and/or add a reverse force proportional to the current speed of the object.
If not using Unreal physics objects, you have to keep a vector for “current velocity” and each tick, slow it down a little bit, and then if the “move foward” is received, and you’re below the max speed, add a little bit of the object forward vector to the velocity.
