Shillelagh doesn't require using Int. You "can" use it, but don't have to.
Ah, I missed that. I guess I don't see much of the point of using it just to change the weapon.
So barbarians can rage with 2d6 in each hand. It's a strong choice for any late game melee.
Or Pact Blade can still use Cha.
True strike requires casting stat.
I don't think Pact of the Blade really works effectively with Shillelagh, there is some stretching to count a mutable pact weapon for the cantrip.
But, more importantly, to use two clubs, with Shillelagh and Rage would require three bonus actions. And for the feat you spent getting Shillelagh, I'm not sure you couldn't do better. Discounting rage cause it is the same for all uses.
6d6+10 --> 31
Taking the Dual-Weapon Style Feat would allow for 3d6+15 --> 25.5 so that isn't enough on its own.
Dual Wielder feat was altered, not sure which version is making it in, but the newest version would allow 2d8+1d6+10
So, the Shillelagh is better than any other single feat dual-wielding, but if we do Great Weapon Master --> 4d6+10+5 -->29, which is almost as good, and has the bonus of giving you an ASI towards a stat you want, unlike shillelagh. So, it is good, but not super broken on its own. And, the benefit of the Great Weapon Master version is it leave open a bonus action for the third attack, beating Shillelagh, and it doesn't cost near as much to set up.
So, Shillelagh is good for a dual-wielding club build, but I'm not sure it is a must have. Just a decent option.
I was wrong. You need EK 7, not 3.
So either Warlock 11 / ek 7 with Lifedrinker, and probably GOO to boost Hex (for your allies).
Or EK 13 / warlock 2, with studied attack and more second winds. Probably levels smoother too.
Either can use Cha for 6 attacks per action.
And you have action surge. Which works because you use it to take the attack action...
Oh... Spirit Shroud works with EB...
So Warlock 11 / EK 7
4d6 + 10 + 4 + 4d10 + 20 + 12d8
= 124
Right, but what is noticeable about that build is that it isn't doing anything new. That character could use a Greatsword for the exact same damage numbers, just with a different set of MAD. Because your Eldritch Knight spells and abilities still work off Intelligence, and to take advantage of the heavy armor you need some strength anyways. And if you are dumping INT and focusing on just strength and Cha... then you end up with the same thing.
The difference is the one hand allowing for a shield. (which you are a level 18 character, a magical item to account for the shield is not impossible) And the weapon mastery differences.
I'm not saying any of these couldn't work or aren't strong choices, just that it all requires a great deal of effort for relatively minor buffs. The strongest thing on the EK build is the fact that they are a Warlock using Spirit Shroud and making six attacks. That has nothing to do with the cantrips at all.
And a straight EK is still doing plenty good, do the standard GWM stuff and you could be hitting for 6d6+3d8+15+5+1d8, with a potential for an additional 4d8 and a possibly 4th attack for 2d6+5, which taken all together is 89 damage, with just feats and wizard cantrips. So I think most of the Shillelagh builds and True strike builds are just getting to the same general end point through different means.