I was going to say you cannot pre-buy items, but you can for ARR and HW relics.

Since you have already done the Zodiac Weapon, I will give a list for the Anima Weapon:

Steps 1+2: you cannot pre farm any items (fate grind/hand in Zeta weapon and then run 10 specific dungeons).
Step 3: This is an item collectathon where you need to hand in specific items to buy the 4 mats you need. Items you can get with poetics are the 4 unidentifiable items you can see, of which you need 10 of each one, each one costs 150 poetics each (which, in total, is 6000 poetics), however, you can obtain these items in other ways, for example, via beast tribe tokens or clearing the floors in Gordias. 3 clears of a floor gives 1 unidentifiable item. You would likely need to be on that step in order for the tokens to trade in to drop however.
Step 4: 5 Aether Oils, 350 each, 1750 poetics for this step, only way to get them (the other method has been removed). This should be your first priority on where to spend your poetics.
Step 5: Hard one as there is some slight RNG to how many you will need. This item is Umbrite. You will need somewhere in the region of 60, but I would short change this so that you don't overspend. These are 75 poetics each and are again the only way to get this item. You will need another item, however, you can only start getting them once you are at that step and there are many many ways to get it that do not involve poetics.
Step 6: 50 singing clusters. 40 poetics each, however, once you get to this step, 2 repeatable quests do open up, one daily which gives 1 SC and one weekly which gives 18.
Step 7: Apart from the light grind, you will need Pneumite, which can be bought with poetics, however you might need to be at that step, alternatively, you can get the items from GC seals. You need 15 which costs 100 poetics each or 4000 GC seals each.
Last step: doing certain trials and that's it. That is your Anima weapon.

For SB, all items can only be obtained in Eureka, so you have to get all items in there, which also requires making progress through the story.

For ShB, again, you can do the whole grind in Bozja, however, there are some steps you can do outside of Bozja, though none require poetics, unless you want to make more than 1 (the first weapon is given to you for free).

For EW, it is just a causality grind. Each step so far has been 1500 causality tomes worth of items, of which you can only buy them when you get to that step. With 3 steps out so far, you require 4500 Causality per weapon.

As for whether the become easier, it depends on what you definition of easier is. ShB relics do have some 1 time steps that you only need to do once ever, but you cannot farm them other steps unless you have a weapon on that step already. For Anima/Zodiac weapons, once you have done one, you still have access to all the vendors and items you need, so you can start making stockpiles of items as you start capping currencies.

Eureka is 'easier' in the sense that you don't have to level again.

EW relics are already just easy, so there isn't much you can do to make them easier.