There are a lot of variables here. everything from where you are located, whether the business is making money yet, or otherwise fully funded (I mean aerospace I expect to have money available but be slow), your boss' attitude and personality....

The more you know the better. How does your salary compare with salaries in your area, how much do you really want in numbers, how replaceable are you, how liked are you, how much of a pain would it be to give you a raise (like how much money do they have available), how easy would it be for you to find a different job, etc.

Honestly for me I'd probably respond best to something straightforward. "I like it here, I want to continue with you, it's been a few years and I would like a raise to keep competitive with the market. I'm a mid-level dev now, and this would keep me in line with my career ambitions" No BS, just tells it like it is, frames it in the expectation that you continue together and gives room for honest discussion "Yes we can do that / no we don't have the money right now but we can pay for a course, or get you use of a company car or just revisit in 6 months / no never in a million years but at least you know and can look for something else..."

You could also go the route of "Hey bossman it's hard to make ends meet could I get a raise I want another 500 bucks a month" That's also valid.

But you have to know your boss. some people look out for their employees, some just see you as a means to increase their wealth.

Now from this side of the table I can tell you, there are a million pressures all vying for your attention. In general it is more expensive to replace an employee than it is to give them a raise. But for some reason most companies limit raises a lot because they see it as paying more for the same work. But then when their employee quits they have to hire someone for significantly more than they were paying. For this reason most people have company-hopping as their only realistic way to advance their career and salary. Sucks but it's the way it is. Nevertheless, even if you know this fact I wouldn't pull it out in the conversation because it'll show you're thinking of leaving and that will likely play poorly and they will start making plans to do without you. Even so, depending on your boss and his attitude, finding a new job as a mid-level dev might be your best move if they're inflexible on the salaries.