Need advices

1. I've never found it to be worth it to farm and store embers unless you're expecting to get a particular servant but that's just my personal playstyle. 5-star servants need an ungodly amount of embers that the second archive can never wholly accommodate, so I've never seen the need to expand it.

2. Farming the 30 AP ember gather and selling the cards is a common method of gathering mana prisms. It's viable. During events, try to grab all the silver EXP all embers and sell them for mana prisms.

i expanded my 2nd archive during the christmas event simply cuz i needed to store away a bunch of saber embers during the 1/2 ap event simply cuz i was going to roll for musashi. if i didn't get her, i was gonna roll for okita. and if i didn't get her, i was gonna roll for umu bride...

i managed to save 320 saber embers, but missed out on musashi (sad face), now i'm on to okita... so well, it was worth it to me cuz i got all that saved up, and plan on doing the same for rider embers for maid baber alter.

I extended mine using the valentine’s mana prisms.

I did it because I ran out of room saving saber emblems for Nero Bride this April/ May. I had 200+ at the time and completely filled the archive even after expanding it. I now have over 300 and will have 375 by the CCC event so she will be lvl 90 on day one.

If you are doing something like this it might be worth it, but otherwise probably not. For most masters saving for the Mona Lisa CE or something like that is almost certainly a better option.

That said, I have no rare prisms (and probably won’t for the foreseeable future) and therefore nothing better to spend the MPs on. And I needed the room....

I find it useful to expand the 2nd archive especially given we don't have any new CE coming in the store (Based on what other people said in the forum). I also tend to keep a fair bit of CE's so I do need a fair bit of space.

As for the 3 star ember, just burn them as suggested by other people, you always need MP and never come to a point you don't need them (At least in my case)

I also find it quite useful to store a few gold ember in the archive even I don't have any servants I like to levelled. But whenever I need to power level a servant, I like have the material handy.
For example, when I was going through Camelot, I had difficulty fighting Gawain and my Euryale was level 1 at the time as well.

I ended up power level her to Lv 70 instantly with 6/6/6 and max Fou because I have the materials handy. This can be said for any future servants you want to pull and the last thing you want is that you pulled your favourite servants, but they are sitting there for months at Lv1 because you have 0 resource.

Expanded my for two things.

1) After 2 years I am near to have all characters leveled to maximum and I want to save exp cards just for case.
2) I farmed all vallentine CEs. They are totaly useless and game realy need special space for them so I expand CE archive for them. :)

But I played 2 years without using archive...

Unless you have no Servants that need to be leveled, I think its best to just use up the embers and save your Mana Prisms for the monthly shop stock as well as the shop CEs.

Just do what I do. When the shop gets restocked, buy the embers for sale and KEEP it in your present box. All the embers will only take up ONE slot in the box, and the box has 300 slots and will keep it there for 1 year. As long as you remember to take the oldest first, there's no worry of the game deleting any items that are too old. Though leaving it in there for over a year seems pretty silly to me XP

I still have about 400 embers in my box saved up the past few months. It's way better than the archive.