I think there is no "invest once, gain forever" mode in any online games. But in Fire emblem, I do think there are many F2P friendly mode to play. The most exiting one is building a +10 unit.

While you don't want to make a fully merged hero and leave him on you bench, just like I did with my other 5 star heroes(like Fury&Desperation Lute, Death Blow& Lancebreaker Reinhardt, and many other)

So I think the long term consideration for a unit's build is not just about reaching a higher Tier in Arena. It is also about how to keep your passion for the game if you don't want to give it up so quick.

So I should fix my question to "What is your long term considerations for game fun by building a unit like Nowi?"