Speaking generally, most of the points you make there unfortunately aren't worth a great deal of salt in a salary negotiation:

• You have half a year's commercial experience, which is very little in the grand scheme of things. You can try to dress that up all you like by ranking yourself against other junior employees, but no-one in charge of salary decisions is likely to be swayed by that.
• If salary hasn't changed for 2-3 years in the entire company of 200k employees, then they're not going to bump it up on the say so of a junior dev. If your salary hadn't changed for 2-3 years that'd be different, and would be something worthwhile in pointing out (but since you've only been there 6 months that can't be the case.)
• Objectively speaking, if project X is only being worked on by a junior dev with 6 months experience and no-one else, then they can't really care about X that much.

However, you mention an important fact in a comment:

My contract ends 31.09.

As per your comment, your contract ends at the end of September - that's less than a week at the time of writing. YMMV, but in my experience, companies sort out contract renewals a lot sooner than this if they want to keep you (think months in advance.)

If you haven't yet approached them about contract renewal at this point, and still really want to renew your contract ASAP, you're not really in a position to negotiate on salary at all.

Speaking generally, most of the points you make there unfortunately aren't worth a great deal of salt in a salary negotiation:

• You have half a year's commercial experience, which is very little in the grand scheme of things. You can try to dress that up all you like by ranking yourself against other junior employees, but no-one in charge of salary decisions is likely to be swayed by that.
• If salary hasn't changed for 2-3 years in the entire company of 200k employees, then they're not going to bump it up on the say so of a junior dev. If your salary hadn't changed for 2-3 years that'd be different, and would be something worthwhile in pointing out (but since you've only been there 6 months that can't be the case.)
• Objectively speaking, if project X is only being worked on by a junior dev with 6 months experience and no-one else, then they can't really care about X that much.

However, you mention an important fact in a comment:

My contract ends 31.09.

That's less than a week at the time of writing. YMMV, but in my experience, companies sort out contract renewals a lot sooner than this if they want to keep you (think months in advance.)

If you haven't yet approached them about contract renewal at this point, and still really want to renew your contract ASAP, you're not really in a position to negotiate on salary at all.