For completion sake, Simple Test table from SR5:

`difficulty threshold`

Easy 1

Average 2

Hard 4

Very Hard 6

Extreme 8–10

As you already noted, for Extended Tests Average hits (as well when using the Buying Hits optional rule) mathematically grows roughly with the square of the dice pool which mean that Simple Tests in SR5 typically required smaller Dice Pools compared to Extended Tests for lower Difficulty Level and higher Dice Pools compared to Extended Tests for higher Difficulty Levels.

This might actually be intended if you also include Edge in the equation. As Edge only affect one single instance in an Extended Test, Edge will have a much higher impact on lower Difficulty Level Extended Tests than higher Difficulty Levels Extended Tests.

If we were to assume that it was intended then this is how an Extended Threshold table could look like in SR6:

`Descriptive Difficulty Simple Extended`

Simple 1 6

More Complex than Simple 2 12

Normal starting point 3 15

More difficult 4 18

Tricky 5 21

Elite 6 24

Standing out among elite 7 27

Having said that, most Extended Tests examples in the book have a Threshold of 4, 5, 6, 8, 10, 12 (and one case of 20) which seem to suggest that the table should start with a lower Threshold than 6 for an Extended Test of "Simple" Difficulty Level.