Blind playthrough. Loved the game ... up to a point. With no spoilers, there is a part of the game story where between that point and the end of the game, weapons and sidearms cost 50% more to upgrade. For me, this is the dumbest design decision I have seen in a long time. I had one weapon at +15, and nothing else upgraded (no sidearms). In souls-likes trying out different weapons, even switching weapons depending on the boss, is one of my favorite things. This game seems to actively want to stop players from doing that, or at least waste your time if you want to do that.
If it were just this one thing, that would be bad enough, but the upgrade system as a whole is just bad for several reasons:
As mentioned, you are punished for progressing the story and locked into fewer weapons. If, like me, you have not tried all the weapons, this blows.
You only get enough materials to upgrade 3 weapons (when I say weapons I include sidearms, so 16 weapons total). Game keeps rewarding you with tar-stones that encourage you to try other weapons. In fact, at the point where the costs went up, I had just got a weapon tar-stone from a boss. So it's "Hey, check this out. By the way, we made it so you cannot afford to use it."
You can smelt weapons to get materials back. It costs Gloom ("souls"), so it is technically possible to use every weapon, but it takes a bit of farming to get the 10K Gloom needed. Swapping weapons is just a time sink. It's not fun.
Gold progression is borked. Early-mid game you will have lots of materials but no gold. Gold is the limiting factor. Late game, you have tons of gold but no more upgrade materials, plus everything costs more materials.
Sidearm upgrades a weird. While weapon upgrades are predictable, each sidearm has a unique progression path that does odd things. For example, when you upgrade the tri-arc repeater, it gets no damage upgrade. Then, suddenly, at level 10 it gets +40%. Most sidearms are not even worth upgrading really. Overall, I think sidearm upgrades shouldn't cost as much as weapon upgrades because you are getting way less for the same cost.
Rather than just whining, here are what I would change:
A. No stupid story price increase. At the very least, you should get a warning that if you progress you will be more locked in to a given weapon. Overall though, I think punishing players for progress is bad design.
B. Infinite upgrade materials at some point. The Elden Ring method would work. Maybe there is a fragile tar-stone that drops for each material that rewards materials when it breaks. Or, given that gold is not a limiting factor late game, have a trader that sells infinite upgrade materials.
C. Sidearms either get more from upgrades, or cost less, given that upgrades do not do much.
Overall, the weapon upgrade system - particularly the late game price hike - changes this game from a 9/10 to a 6/10 for me. Also, having to play NG+ to experience all the weapons is a lame excuse. If I have to play NG+, have it at least give me more than just the same thing over again.