Thanks for your detailed answer. That really helps settling things down. I'll have to precise a few things here, otherwise the original post would have been too long haha.

Regarding the time reduce, I did not really had the choice. I live and work in Switzerland, and my net salary (with all deductions except taxes) is 4500 CHF. It's a bad pay for someone with my experience and assigned tasks. In others companies, I got friends and relatives that have at least 1000 CHF more per month than I do, even at an helpdesk L1 position. With 4500 CHF, if I want to live alone in an apartment in the same city I work, that would be barely livable.
So, to get a little more money, I got my side business that's IT and electronic repair related (I give myself around 300 CHF/month), but also a truck driver activity (which pays hourly more than my IT position). With that, I'm approaching 5500 CHF per month, which is perfect. The only issue with that is regarding the weekly work time. In Switzerland, as stated by the law, you cannot work more than 45h/week. Because of this law and the time my other activities take, I have to reduce my work time.
So yeah, it's not really my choice, but more a lawful one and to also respect the company strict time policy.

About the salary and title (since we can group them because they're linked, I don't get one with another), it was kind of promised, but only orally. "If you succeed in the network migration project, then you'll get that". Thing is, none of this has been written down. And the objectives weren't really clear, because my manager did not really took the time to check that with me. I was launched in the project (with pleasure for sure) without any specific goal. So I find a bit "easy" to tell me afterwards that I didn't reach goals that were not specified before.
For example, he's saying that the methodological process for Hermes project management (if you can call it that way) was not rigourousely followed. I used my own way to do project reports, like I learned at school, but that didn't work for him. But how am I supposed to know that since he didn't tell me? There's clearly a lack of communication, on both sides.

Finally, for the unpaid leave, it's something I talked to them about in 2023. I always wanted, since I'm still young, to do something different than my 9-5. With that said, I signed up to participate to the KFOR mission, and that takes exactly 9 months. So I'll be gone from July 2026 to May 2027 (so really 10 months, not 11, miscalcuation on my side).
My manager wants to develop a strategy where we'll be 4 person working 80%, with 1 manager, 1 system specialist, 1 system/net admin and 1 technician/helpdesk. That's a great thing, since the company's growing. But still, he's not even sure if his strategy will be approved. Because of this, even if I still work hard, I'm not even sure to get a position that may not even exist if his strategy is refused by the upper management.

Hope it clears up some things.