The job is genuinely fine and I am bored out of my mind, is that a real reason to leave?

It is a real reason, and treating it as ingratitude is what keeps people in a role for four more years than they meant to.

Boredom in the sense you describe is not a mood, it is information: your capability has outgrown the work. That has consequences beyond how the week feels.

Skills decay relative to the market. Eighteen months of variations on work you already knew is eighteen months where others moved. This compounds, and it is the part that is invisible until you are interviewing.

Your options narrow. The longer you do only what you can already do, the more your next move looks like the same job elsewhere.

It rarely stays neutral. People describing exactly your situation usually describe resentment a year later.

So the question is not whether it is a sufficient reason. It is what specifically is missing, because that determines whether leaving is the fix.

Be precise about it. Is it that the problems are too easy? That there is nobody to learn from? That there is no scope to own something bigger? That the technology is old? Those have different answers, and two of them are frequently solvable where you are.

Work that out before you start applying, or you will optimise for the wrong thing and be here again in two years.

I have fixed this twice without leaving, so it is worth trying before you go: partly because it might work and partly because it is a much better position from which to leave if it does not.

What actually worked:

Ask for a specific harder thing, by name. Not more challenge: the migration nobody wants, the system with no owner, the thing that keeps breaking. Managers are usually delighted; capable people volunteering for the hard unowned thing is rare.

Move internally. A different team is often a different job entirely, with none of the cost of leaving - you keep the relationships, the context and the tenure.

Take on something adjacent. Mentoring, hiring, being the person who does the thing nobody has done yet. This changes the shape of the week rather than the content.

Say the words out loud to your manager. Not as a threat, as a fact. I am not learning and I want to. A decent manager would rather solve that than replace you, and yours sounds decent.

Give it a defined period, a quarter - with something concrete agreed. If nothing changes in that quarter, you have your answer and you have it with evidence rather than a feeling. That is a much easier decision to act on.

The honest version from someone who did leave a comfortable job for a more interesting one.

What I gained: the work was genuinely harder and I learned more in six months than in the previous two years. That part was real and I would do it again.

What I gave up, and underestimated: knowing everybody, knowing how things worked, being the person people asked. All of that resets to zero and it takes the better part of a year to rebuild. For the first three months I was slower and less useful than I had been in a decade, which was uncomfortable in a way I had not anticipated.

What I did not get: the feeling of never being bored again. New work is interesting for a while and then becomes work. If you are hoping the boredom is permanently solved by a change of employer, it is not - you are buying a couple of good years and then the same question returns.

So my honest advice: try the internal route above first, because it gets you most of the benefit for none of the reset. But if it does not work, do go. The version of this that ends badly is not leaving too soon; it is staying five years because it was fine, and finding out what that cost when you eventually have to move anyway.