I have the following reasons I'm applying for this particular position
- all of them are true, but should I mention all of them?
It's good that they are all true, since it is best to always stick to the truth. But some of these are "safer" to discuss than others in an interview.
My current position involves engineering in a sales back-office role -
I want to change to a more technical position
Wanting "a more technical position" is a very valid reason for a job change. This is a good area to discuss. Be prepared to talk about what this means to you. Focus on the parts of the new position that excite you, but don't dwell on the parts of the old position that you don't like - some of those may still apply (hopefully in lesser amounts) in the new position.
My current position demands quick work, and it is expected to fudge
over details to make a deadline. I want to work more thorough.
Tread carefully here. When I read this, it comes across as "I don't want to work hard" or "I can't meet deadlines". I would avoid this as a reason for changing jobs. Do you really think your next job will never require any quick work or deadlines?
My last position was mostly X, where the position you are offereing is
mostly Y - a subset of X. I look forward to deepen my expereince of Y.
Your organisation also does X. Maybe, a few years down the road,
there's a possibility of me doing X again - I'd like to talk about
Going deeper in one area is a reasonable reason to change. Again try to emphasize the "Y-ness" that you see in the new job, but don't dwell on X - particularly if you see some X in the new organization.
I don't mind overtime, but in my current position overtime is handed
informally and is often uncompensated. As you are public service, and
where voted "best employer" multiple times, I expect an arrangement
that's fair for all sides here.
Unless the lack of strict rules about overtime are a deal-breaker for you, this is not something I would bring up in an interview. If you are interviewing for a salaried position, you will seldom find such strict rules.
People who interview with me and who spend any time dwelling on overtime never get the job. While the jobs I offer in my current company have very little in the way of overtime requirements, it still does happen occasionally, and as salaried employees, it is uncompensated. If that were a big deal with a candidate, then I can't hire them.
You need to decide how important overtime is to you. Then you need to measure that against the kind of corporate culture you see in the hiring company. Expecting some sort of special arrangement just for you may be unrealistic.
Overall - determine what factors are most important to you as you transition to a new job and spend time discussing those, and asking questions in those areas. They want to make sure you are a good fit for the position, you want to make sure they meet your needs.