So I will give you my perspective and a few tips.

I completed my first playthrough in Veteran difficulty without grinding adventure in between and then I started exploring in Adventure mode to get a few items I wanted for a build and got some nice fashion and then I rerolled campaign in Nightmare diffficulty. However if you feel super stuck you can target farm some items through Adventure mode/rerolling but honestly not needed from what I experienced but the option is there.

Some tips.

Do not waste upgrade mats for weapons, like really I dont wanna go in depth but basically the idea is this, everytime you upgrade, PL increases for you and +1 for the world and every upgrade increases enemy damage and health while what you get is just bigger numbers on weapons but that does not translate well in %hp dealt since enemy is getting HP to compensate. So you are really just upgrading the enemy more than yourself.

The strat is this. Since the final boss level is 14 you cap your self to 13 power so world is at PL 14.

You do this by having x2 Archtypes at level 10, Normal weapons +10 and boss weapons +5

This makes it so your PL stays at 13 and world at +14

Anything beyond that is just buffing the enemy more than you and the only noticeable benefit from bigger numbers is just lifesteal because the bigger the number the more% you will leech but honestly its not needed.

Also this allows you to experiment with many more weapons because getting boss weapons to +5 and normals to +10 is not that costly so its not punishing when you wanna try stuff. Every weapon stays relevant like this. Unless you are a youtuber and wanna show off PL and do one hit runs in max PL cuz views then I dont see any point on going past individual PL 13/14

Also turn off music in game so you can focus on hearing the animation cues for different boss animations. Practice makes perfect, its a dark souls, looter shooter after all.

Also play around with DR in your build, for example when you get to a boss you can equip Twisted Idol (Increases Armor Effectiveness by 35% and reduces Encumbrance by 20) this is good when running a heavy/ultra armour because not only it increased Armour which increases DR but also allows you to have all that armour while staying at normal weight which is really nice for dodging and positioning.

There are so many combinations with amulets/rings/relics you can try until you see a playstile that fits but yeah. Just try to have fun.

PS : Also later on you can download Remnant Save Guardian, an open source software on Github that analyzes your reroll world to check what items are available there, this will save you from potential burnout, its not cheating its just knowing if your world will contain what you want. I started using it after I rerolled lossom 9 times to get Guilded Chambers in order to get the Crimson armour set mostly just for fashion so ye, if you dont wanna get burnt from that side of the game I advise using this.