'My aim in this job is to be a manager.' Or 'My aim in this job is being a manager.'
'To be a manager in this job is difficult.' Or 'Being a manager in this job is difficult'
The blue ones are preferable.
But there's a semantic problem in them all. In the first pair, if you are not a manager, you can't aim to be that in that job. If you change 'be' to 'become' it would make more sense. Again, in the second pair, being a manager is the job. -> "Being a manager in this job of manager is difficult." Awkward. You could change 'job' for company, field, industry ...