In play a level 3 Imp is still a pretty good melee combatant because of invisibility and the fact they use their reaction to attack. So your invisible Imp attacks using a reaction then turns invisible again using its action. Being Invisible eliminates AOOs and makes it difficult to target and damage the Imp.
If it was not for the Invisibility I would agree that they would not be as good as the better level 3 melee characters.
I agree. There are always going to be levels where one character's powers are a bit better.
In that context, I prefer something like the 3E D&D Wizard's familiar that scales with the Wizard's level.
A5E Pets & Sidekicks has a somewhat similar mechanism for scaling pets. This does make for a fairly powerful familiar. To balance that, it is a significant investment for an A5E Warlock to get that kind of scaling - you need to take Pact of the Chain, Frog Fangs eldritch invocation, and Empowered Familiar eldritch invocation.
There is an action economy issue, which is the crux of this thread. What does the Warlock need to pay for the familiar to attack?
In D&D 2024, with the Investment of the Chain Master eldritch invocation, you can command your familiar to take the Attack action as your bonus action; and Pact of the Chain also allows you to forgo an attack to have the familiar attack by using its reaction. So if I am reading this correctly, a level 5 Warlock could have their familiar attack twice: by using the Warlock's bonus action, and by forgoing one of the Warlock's attacks.
This can be combined with Pact of the Blade and Thirsting Blade to have the Warlock also attack once at level 5; or you can pull this off with a level dip into say Paladin for Nick weapon mastery if we permit the ruling proposed in this thread. So a Warlock 1/Paladin 1 could have the familiar attack and the Warlock attack if we permit the Nick attack to be replaced; and a Warlock 5/Paladin 1 could have the familiar attack twice and the Warlock also attack. A Fighter level dip is also strong, particularly if done at character level 1, as that gets Constitution saving throw proficiency, heavy armor proficiency, fighting style, and Action Surge at Fighter level 2 if you take it.
Given that Fighter/Warlock and Paladin/Warlock already have strong synergies with Warlock, I understand the reluctance to allow the Nick weapon attack to be substituted by the familiar's attack. Given that the Warlock can already have the familiar attack on a bonus action using Investment of the Chain Master, I have changed my mind and would rule no on allowing Nick to be substituted. It seems unnecessary extra power creep to me and a somewhat dubious violation of the rules as intended (RAI).