> it is up to me to mentor and guide these developers
That right there is what makes you a senior. Make sure your work, your performance, and your track-record in levelling up your coworkers is well-documented, and use that to justify your bump and bonus. There is a chance that it doesn't work - in that case you have a year (or more) of experience with leading teams and mentoring coworkers that you can take to your next gig.
With regards to either or not things are fair, it is very possible that the business doesn't understand the value of the work your team does. It might be worth finding out what it is they do value and either trying to provide it or - if you can't directly provide it - enabling those who do provide it.
In either case, since you are going to be at this job for a while, it's a good opportunity to focus on fattening up your resume with those soft skills that often don't come easy but are valuable.