I'm burnout and have been questioning myself after 2 years servicing clients in the company I'm in if I'm good enough for other roles (using as example) promoted in HackerNews, or whatever job portal.

When I spend so much time idling by and letting that frustration build up... I just start questioning if I'll be able to do other roles and not fail the team or whatever company I'll be working for because of how dormant I am.

It's not like I knew a lot. I don't consider myself to be super talented, just the average guy trying to make it out there but yea... I need to look into moving on.

My advice is to interview a couple times (with companies you don't mind missing out on), find out what you are missing and work on side projects that interest you. Then interview again about 6 months later. There is nothing like interviews to set your mind straight. Competition and hard questions seem to energize my "survival" instinct.

...doing something hard and outside of your comfort zone I guess is what I am recommending.

I was in the same situation as you at my previous job, feeling like I wasn’t learning anything because I was working on a legacy system where every design and business decision made was pure insanity

After five years at this place I finally got my shit together and applied for a job

I spent the job interview asking pointed questions about their infrastructure and how they ran their business and then I contrasted their way of doing things with my old job and explained why their way was so much better

Turns out I was learning! And learning how NOT to do things carries some value

Invest time into thinking about and listing specific things you've learned (technical, but also organizational, managerial, etc.) from maintaining this horrible legacy system.

This list will show you that it hasn't been a complete waste of your time, and it will show future employers that you're thoughtful, observant, respectful of your own time, and healthily critical.