If your problem were only with the single word which, it would be routine to solve it by simply using a different word:
I have been working for the past seven years as some form of interface designer, making me a practitioner in the field of usability studies.
But there are other ways to avoid the problem you have identified. I do not think that lies with the word which, however, but in the way you construct your statements.
The basic architecture I see at work here is statement of fact + explanation of statement. That is what gets to be tiring. So I suggest you try to vary your delivery of information. Some examples follow, which I will model on your first example (note necessary insertion of comma):
For the past seven years or so I’ve been working as some form of interface designer[,] which makes me a practitioner in the field of usability studies.
Lead with the result as an independent sentence
I am a practitioner in the field of usability studies. For the past seven years I've been working as some form of interface designer.
Lead with the result as a subordinate clause
A practitioner in the field of usability studies, I have been working for the past seven years as some form of interface designer.
Blend the two statements into one
I am a practitioner in the field of usability studies who has spent the past seven years working as an interface designer.
(Note that in the last case I have removed "some form of" as a modifier for "interface designer," because it weakens the statement.)
Those are just three examples, and all are based on the statement you yourself made. But it is not the only statement possible. You can talk about other things, you can lead into something obliquely, then quicken the pace to make a bolder statement, and so on. The important thought that I would leave you with is that varying your prose is always a good idea. You can't bore people into reading what you write.