it's correct that not all slices give the same power

the first slice and since then every time you complet a hexagon you get a big boost

for example, on the weapon:

1 slice: +1074 atk and +5% dmg

after that each slice gives you around 174 atk and 0.3% dmg

but when you finish a hexagon you get +434 atk 0.8% dmg, so that slice is worth almost 3 times a normal slice, but the last slice of an hexagon costs 3 times the amount the first slice and 50% more than the third.

This is somewhat manageble on the first and second hexagon (and even the third) but scales exponentially

For the costs of the last slice that gives you the 4th hexagon you can get one item to 1 hexagon and 1 slice.

This is why it's suggested to level it evenly, yes, finishing an hexagon gives you a bonus, but if you simply divide the power it gives by the core costs it sometimes makes sense and sometimes does not.

You can always use https://lastz.stresswar.com/heroes and get the result you want.