All of which is true; and to add on to your points re: Aku's competence,
I sometimes see people call him an incompetent cartoon villain because he talks too much, runs away, or sends henchmen after Jack.
Do you know who talks too much? Jack. Aku escapes in ep 1 because Jack was bantering instead of killing him & Jack wins in the last ep by not stopping to talk.
When has talking too much ever caused a problem for Aku?
MAYBE the last ep? But his mistake is only 1/2 gloating before killing Jack; the other 1/2 is procrastinating the kill too long.
90% of the time he's "talking too much," he's talking to himself, at home, alone. When he's talking to somebody, he isn't distracted—he immediately reacts to threats and attacks. (He'll even cut himself off mid-evil laugh just because he thinks he might sense something amiss. Keen situational awareness.) He's never accidentally given away important info while monologuing. He's never lost a battle due to pausing to gloat. He's never had an enemy escape a trap because he was busy explaining his evil plan.
So in what way, exactly, does talking "too much" impact his competence as a villain?
THIS IS PROOF THAT AKU'S A SMART VILLAIN.
Outside Jack finding a portal (which you addressed), there's ONLY ONE WAY Aku can lose: if Jack stabs him a lot. That's it.
And so, Aku's most reasonable long-term strategy is... making sure Jack can't stab him a lot. Whether or not Aku wins against Jack, as long as he stays out of melee range, he'll never lose.
Plus, look at how their battles go. In all their fights, Aku only draws blood once. Jack's sword carves through Aku like a flamethrower through tissue paper. The one time Aku has the upper hand during a battle is when he's got Jack's sword.
Aku's not a bad fighter, as we see in Warrior Woman, Zombies, and Jack vs Aku. But his skills and powers are useless when Jack's pointing the sword at him. So like, what would happen if Aku kept fighting and DIDN'T run away?
If somebody drops a 5 ton safe over your head, are you a coward if you step out of the way rather than try to catch it? Is catching it courageous and honorable? No. Catching it is stupid. Stepping out of the way is the absolute minimum threshold of common sense. Aku has common sense.
You know what an incompetent villain would do? "I'll fight him for I am unstoppable and all-powerful! He cannot possibly defeat me!" or some arrogant BS like that. Aku openly admits more than once that he can't beat the sword and that's why he won't fight it.
or sends henchmen after Jack.
(road work ahead voice) Uh, yeah? I sure hope he does. And the alternative is... what?
Here's the thing about henchmen: they're not Aku! And therefore, they're expendable! If you're playing chess and wanna take out the enemy queen, it's better to lose every single knight, bishop, rook, and pawn on the board than to lose the fucking king. If you lose the fucking king the game's over.
Once again, the fact that Aku sends other people after Jack—and in fact, immediately sets things up so that people will autonomously go after Jack—is proof that he's a reasonable and intelligent villain, not an incompetent one.
You know what an incompetent villain would do? "He's my nemesis! No one's allowed to kill him but me!" Aku does not give a shit who does it, he just wants the guy dead.
tl;dr - even if Aku hadn't achieved his main goal of destroying all the time portals, most of the reasons people accuse him of being a bad villain are either irrelevant or actually the exact reasons he's a very good one.
