The optimal way to balance the imp is to make it castable once per long rest. Other details, such as damage, hp, and infinite roam range are another topic that the developers can decide on.
The infinite roam is an actual warlock invocation, and the familiar should be tethered without it. As an addition, I'd say the first cast be free, but if the familiar dies, it should cost the same amount of gold (or materials) it does in the 5e PHB (or an amount more conducive to the in game economy of BG3). Summoning and dismissing the familiar should only cost an action, until it dies. This is how it works 5e. It also should not disappear on a long rest, as it does now.
As it stands, taking a long rest has no drawbacks, so if your familiar dies, simply take a long rest and get the familiar back. This is pointless for now and brings up and entirely different problem altogether.
