Thoughts:
1. That would be a figurative tank. Make the character a Leopard 2.
2. Strong from start to end can work if action isn't the main genre. If the main genre is fantasy/sci-fi focus on the character's observations of the world their in and their unique perspective from the peak. If the main genre is drama or comedy focus on the character itself and his/hers interactions with other characters. Internal and external conflict for drama, and gags for comedy.
3. Focus on character flaws, not physical flaws. You've already made it clear you're idea for this is similar to how Goku is set up, although you see it as a problem. Since you want the character to be super powerful I fear you'll fall into the same trap Toriyama has. You'll most likely keep writing a villain to be the one guy stronger than the character and then the main character beats them in time to swap to a new arc villain that's the one guy stronger than the character. This makes the story repetitive so don't bother with making the conflict derive from a lack of strength if the character is already strong.
4. Don't make the character the main character. If the character is a villain, a goal for the main character to reach, a mysterious figure, or a mentor then overwhelming strength is actually a benefit to the character, because all of those characters push the main character's growth. If the character is actively fighting on the side of the main character or is the main character, it just removes stakes from the story.
5. Don't put the character in a group. It's a stupid lesson to learn that you need to cooperate with people when you're that much stronger than them. If the character is indestructible then all of his/her plans would be ones in which the very much non-indestructible teammates get killed.
6. Don't write the character to only be able to defend. If the character is incredibly physically tough they'd also pack a punch. If you make it so that they defend themselves from all the attacks but need someone else to finish the fight, then you might as well just focus on the character that's finishing all the fights.
7. If you don't listen to any of that advice, make the character a strategist so that they need to come up with a new plan to win each new battle with different characters that possess different advantages and disadvantages each time. This will at the very least add variety to the story, keeping it engaging.
2. Action isn't the main focus of the series, but is an integral part of the series, so there would be lots of action scenes, but would focus and mostly center round the world view and morals the various characters have and how that affects them and how the difference in their world view affected them while growing up.
3. For the Goku part that was merely an allegory the character isn't actually like that and there won't be a villain stronger than the rest just for the tank to beat it's more of a series of people in their way that they have to take care of all varying in power levels, also the tank I'll be focusing on is a villain in a way that would do anything to achieve their goal, lives means or making sure people that aren't part of the objective are okay completely pointless to them and would even kill any by standards that obstruct it.
4. The stakes isn't so much of the battle element rather on the mystery of the secret organization that they run and the drama, action is there cause magic and fantasy, the tank is a main character of sorts, but still on the villain side of things.
5. They didn't and do actually want to be in a team, okay this is hard to explain, but them being in a group is all part of the process of the character arc, they aren't actually super strong; they are incapable of taking damage from most people in the verse, but pack a huge punch thus more than not require partners to aid them, so the non-indestructible teammates are actually needed.
6. The tank won't be the main focus of the series, but they would be the main driving force for most of the arcs and yes they can mostly only defend cause most of the power is only in defence, doesn't mean that there ways to finish them as is and finishers would have time to shine.
7. I do intend to make them a strategist that comes with lots of battle strategies and contingencies.
View attachment 18258
What I think of tanks.
Yes, except for I think that Spidey would take permanent damage if he physically attacks my tank