Regardless, I would suggest that you watch your language and not say "proper job". People might say the same thing about server maintenance, and then how would
you feel?
I have done research online and while some people are passionate about the job the majority of individuals seem to agree that it's a job with poor pay and little chance to create a more substantial career.
Did your research also show that one of the main reasons for that is not the fact that eikaiwa and ALT jobs are fairly dead end, but that to move up to higher paid jobs (or jobs that command more respect from some people), you have to have a degree and qualifications more than what entry level eikaiwa demands? That takes time and money, and people your age (and slightly older) are the ones who find themselves in that situation a few years after they've moved here and have decided they want to stay and move "up", but so many are not willing to put in what it takes. For university jobs nowadays, even for PT, a bachelor's degree and language skills and experience aren't enough; you also need publications, and that's a big barrier to people who never even knew what a research journal was before they came with their BA in geology in hand.
To answer your original question, having JLPT1 is an excellent thing to help get a job here. I see you are 28 and are in IT, but do you have a college degree in that field? You'll need it for the work visa. If you want to start here by becoming an ALT then shift to IT work, you'll have to get a different visa for both position. That's not hard to do as long as you have the qualifications, and for non-teaching jobs that means a "proper" college degree.
is it very hard from within Japan to find a decent job?
The difficulty lies in many factors pretty much as it does where you live now. Skill set, experience, degree, language fluency all enter into play here, but so does your timing. Most jobs in Japan begin the fiscal year in April, just so you know. That means hiring will often take place 1-3 months previously. There may be a slight surge in early fall.
I'll be satisfied if I end up working in translation (Ideally IT)
I don't see the connection between translating Japanese docs to English ( or vice versa) and doing server maintenance. Can you explain what you meant? Moreover, translating is really boring lonely work, so do you really know what you'd be getting into?