I want to say that in the past I decided something that I will do in the future:
Already in my bachelor years I decided that I want work at [. . .] after my master.
Is want here correct, or should it be written in another tense?
For consistency, it should be "I decided that I would want work at...".
"I want work at..." is the present tense (ie. it implies that I currently want work there) - "I decided that I would want work at..." implies that at that time in the past, I made up my mind that at some point in the future, I would want work there.
The sentence is an example of indirect speech (or indirect thought). In direct speech it would be written:
Other verbs of thinking or saying can be substituted for decided:
When transferring these thoughts to indirect speech, it is usual to backshift the verb in the direct speech clause:
Note: It is permissible to retain the present tense in indirect speech, if the verb (in this case the 'wanting') still applies:
However, in the OP's sentence it is clear that the deciding and the wanting are well in the past, and hence the past tense is required for both verbs.