Edit: Since he is under contract for 2019 already, perhaps an extension would change to something like this:
2019 cap hit - 800k
2020 cap hit - 17 million
2021 cap hit - 21 million
2022 cap hit - 25 million
2023 cap hit - 25 million (12 dead 13 cap savings)
2024 cap hit - 27 million
Maybe even give him a higher 2019 cap hit and lower some of the future year's cap hits, if possible. For example, his 2023 cap hit would remain the same, but if he is released, he would only count 4 million as dead money and 21 million in cap savings.
Sorry, I am not a cap expert.
16 passers currently average over $20M per season, this grouping also averages over $49.27M in guaranteed money.
The contract can be for 5 years after 2019, the final year of Dak's rookie contract, making it a 6 year extension.
The base salaries in '19, 2020 and 2021 would all be fully guaranteed upon signing, ensuring the QB would get a large portion of the money included in the new contract before the team could realistically expect to get out of it, should the need arise. 2022 would be guaranteed for injury and could become fully guaranteed if Dak was still on the roster at the beginning of that league year.
It includes a $25M signing bonus ($5M per yr cap hit X 5yrs), $60M fully guaranteed upon signing, '22 guaranteed for injury and a total value of $128M ($21.3M AAV).
2019: $7M base, $5M signing bonus = $12M cap hit
2020: $12M base, $5M signing bonus = $17M cap hit
2021: $16M base, $5M signing bonus = $21M cap hit
2022: $20M base, $5M signing bonus = $25M cap hit
2023: $22M base, $5M signing bonus = $27M cap hit
2024: $26M base, $0 signing bonus = $26M cap hit