For me, a huge part of what makes weapons feel interesting is their value and opportunity cost - I just like durability - so to me any system that makes items infinite use needs to replicate that same feeling.
Each item should have a meaningful upside and downside so that no one weapon is ‘more’ than any other, and that as much as possible, each item maintains “a use” – but this runs very contradictory to how the general approach of Fire Emblem’s weapon progression actually works, ie; that Iron and Steel are each basically strictly inferior to Silver besides price and maximum uses.
This is directly why Fates feels poor - Iron weapons have no downside, while others do - but the non-Iron weapons’ downsides often overwhelm the other advantages they bring. In contrast, in multiple other games, there’s no penalty associated with using the biggest stick you have, so you do so.
When it comes to how you want the player’s weapon progression to work without durability, I don’t think it matters too much - by just being given more weapons or by letting you trade in old ones and gold for better ones or something entirely different - because what matters most is if I want to actively use all the tools I’ve been given. If I have no reason to use Iron other than because I don’t have enough Steels to go around, there’s not actual choice.
(Now, in FE, there’s times you’d like to deal less damage to distribute EXP as you please; but I feel these are niche enough that it shouldn’t be considered a valid ‘purpose’.)
Rather than “nerfing the weapon” directly, the ideal is more oblique - a focus on “making the choice of which weapon you use matter more”.
Removing durability means a weapon’s rarity-of-acquisition or durability to suit that idea is no longer relevant, so when you give the player something they will always have it forever - and it follows that when that happens it needs to come with a reason to not just glue the item to always being equipped by someone for the next twenty hours of gameplay.
So, for example, if you have a standard trio of weapons, you can’t follow the vanilla layout -
Steel is sometimes better than Iron; but both are obviously worse than Silver.
In vanilla GBA FE, this is made up for with Silvers being rare, expensive, and low uses.
So if you don’t have that as a lever, then the items need to be given different points:
For an example, suppose Iron Swords have 5 might and 90 hit – you could make Silver Swords keep their 13 might, but drag their hit down to 60.
This is a terrible example, naturally, since with too-low hit rates an item is effectively unusable, and that threshold is highly variable.
The other thing to consider is that they don’t need to be balanced. There just needs to be something for each to do, so that you are rewarded for remembering and utilizing the tools at your disposal – It might actually be totally fine to have a weapon that is statistically a different weapon but better, as long as it has some additional trait that makes it not be totally for one over the other.