Good idea to take alchemy on impossible?

[Gatts]

Based on what you write, you could be capable to answer your own question:
if you know most or maybe all spells, you know how much it takes to level them up, you know how much crystals you will probably get... you could prepare you own calculation deck, which may give you your answer =

collect all spells you need/want, count the crystals needed to level them up and think about the difference in crystals (on impossible you start with less crystals, also in the game you may not find as many as on normal)

You can rely on the fan manual if you do not remember the exact costs of the spell upgrades...

as counter metric get the incoming RUNES - and check how much runes you will have from level ups (you get less on impossible I assume, it is again in the fan manual) and how much you need to get normal skills- like chaos magic, order magic...

I used to get to Chaos level 3 + higher magic level 1, then I invested all further runes into alchemy, so I could have larger variety of spells, also I waited with scribing them into book/leveling them up till I had alchemy 3 - I leveled only really necessary spells till alch 3... I always liked to have more spells, but honestly I did not use most of them often... so I could save leveling them - and not need alchemy at all

in anyway - check the fan manual, read through all the spells you want - make some calculations