Learn Croatian: Vjetar je bio toliko jak da je iščupao stablo u dvorištu. - The wind was so strong that it uprooted the tree in the yard.

Yes, both toliko jak da and tako jak da are correct and common in Croatian when setting up a consecutive (result) clause. Using toliko (so much / to such an extent) puts slightly more emphasis on the measurable intensity of the wind's strength, perfectly setting up the extreme result in the da clause.

The verb iščupao means 'uprooted'. How is this word formed?

It comes from the imperfective verb čupati, which means to pull, pluck, or tear at something (like pulling weeds). The prefix is- (which becomes iš- before the letter č due to sound changes) adds the meaning of 'out'. Together, this forms the perfective verb iščupati (to pull out completely, or uproot).

Why does the sentence use the indefinite adjective jak rather than the definite form jaki?

Because it is used as a predicate adjective here, following the verb 'to be' (je bio). In Croatian, when an adjective describes the subject through a linking verb rather than sitting directly next to a noun, it must take the indefinite form if one exists.

Why is there no pronoun like on (he/it) before je iščupao in the second clause?