The talents usually state explicitely that they can be purchased multiple times for different choices .. so basically "it depends".

if you look at the following links: https://dragonage.fandom.com/wiki/Spells_(Dragon_Age_II) https://dragonage.fandom.com/wiki/Spells_(Origins) https://dragonage.fandom.com/wiki/Abilities_(Inquisition)#Mage_Abilities and finally this: https://wondersofthedaspodcast.wordpress.com/resources-for-the-dragon-age-rpg/spell-tree-charts/ you can build up talent-trees ... some more easiely ... some need more work. for example: just keep stress spending and the skill roll at standard difficulty. T

i did not read very deep into the dragon age rules ... ... but arent the magic schools of DA more suited as Talent-trees rather than anything freeform ?

Since not all races provide the benefit of an additional career skill and/or additional ranks, you cannot generalize the issue. In the case of the Deep Elves, the skill is a special feature and taking the Mage career afterwards has no mechanical impact nor benefit as @Swordbreaker already mentioned.