I have got probably 6-7 years of salah where I didn’t pray. Is it possible to make these up by having the intention of making up farz prayers during my nafl prayers (for example my 8 zhur ruka’a nafl, I make intention of making up 2x zhur prayers while praying the nafl)? Or do I actually have to pray the missing prayers with the intention of making them up?

You have to make the intention of the missing prayers with the wajib intention.

Nafl is 2x so 8 rakat would be 4 prayers 2x