Learn Finnish: Me kärsimme, mutta saavutamme silti maaliviivan sisulla. - We are suffering, but we will still reach the finish line with grit.

Yes, they are mostly interchangeable here. However, silti (nevertheless / still) feels slightly stronger in this context because it directly pushes back against a specific obstacle (the suffering). It emphasizes achieving the goal despite the pain, whereas kuitenkin just means however or anyway.

The translation says 'we will reach', but isn't saavutamme in the present tense?

Exactly! Finnish does not have a dedicated future tense. You use the present tense verb saavutamme and rely on the context. Because reaching a finish line is a completed goal, using the present tense naturally translates to the future in English.

Why does maaliviiva (finish line) have the -n ending to become maaliviivan?

It is the object of the verb saavutamme. Because the action will be successfully completed (we will reach the whole finish line), the object takes the genitive-accusative -n ending. If the action were ongoing or incomplete, we would use the partitive case instead.

I learned that the -lla/-llä ending means 'on' or 'at'. Why is it used for sisulla (with grit)?

The adessive case (-lla/-llä) is also used to show the tool, instrument, or method used to do something. In this sentence, sisulla literally means 'by means of grit', which we translate as 'with grit'.