One Warlock invocation is seen as being equivalent to a basic (no-level-prerequisite) feat. Likewise, one feat is seen as equivalent to a no-level-prerequisite Warlock invocation. Hence, there's a certain equivalency there. It's not quite equivalent, due to the enforced limitations, but it's a starting point.
There is an invocation in 5.5e, which gives Extra Attack. It requires a specific class (Warlock) to be 5th level. It becomes 2x Extra Attack (that is, attack three times) at Warlock 11, without needing to invest another invocation. It's Thirsting Blade, and that's all it does.
To me, this says that the baseline feat requiring 5th level is fine, regardless of multiclassing. However, if you wanted to allow higher amounts, I would add...
Improved Extra Attack
Prerequisite: Extra Attack feat, character level 12, 11 levels in at least one single class
When you use the Attack action, you may attack three times instead of only once.
This is less powerful than Thirsting Blade (since it requires two feats, not just one), and requires a high degree of focus. You'll have given up two of the three feats usually available to your character at this level, and you have to have nothing more than a one-level dip to make it work--or wait until even higher levels than 12.
It'd probably still be a little wonky in terms of power, because now you can have Extra Attack on a Rogue. But 5e doesn't really care that much about balance to begin with, and thus helps non-spellcasters far more than it helps spellcasters.