If you have that much code and that many servers in an SMB, your work is likely critical to the company. Assuming your managers are rational and want what's best for the company, it seems like they might taking you for granted and that they are overlooking a fairly big risk (you leaving).
It's possible that you have done a "too good" job and left them with the impression that your job isn't that hard, and that you could easily be replaced. Things are rarely broken, new features are added on time, and nobody feels the urgency of scaling your one-person team.
I'd use your qualities to communicate these things. Be the employee you wish you had. If that's not appreciated, take the first chance you get to leave.
You're loyal, so perhaps you could proactively list what it is that you do in your job - "in case I get hit by a bus". Show that you care about the company, and that you're trying to help them prevent a bad situation. List everything you do and have done that provides value for the company and what skillset is required for those things. Writing your job description like that will give a better picture of the scope and importance of your work. I assume your managers aren't very technical, so it also gives them an opportunity to check with their advisors and network, in case they think you're exaggerating (don't be too humble, and be specific).
You're also hard-working, so one approach is to tell your closest manager that you think your market value is X, and that your current compensation is too far from that to justify you staying with the company in the long run. "I want to stay at Company, but I can't afford not to switch jobs eventually since working here effectively costs me Y per year."
If that's not getting across, I think that the company needs to feel the pain a bit. See if you can find a way to leave while being as professional as you can. Set the date and give them time to figure out what to do. They will most likely have trouble finding a replacement, and if they do they will not have a lot of time to on-board. You've warned them, been firm but professional the whole time, so now it's your closest managers responsibility. The higher ups, if you have any, will not be pleased that they weren't warned in time.
You don't owe them anything, you're valuable and you have knowledge that will be expensive to replace. If they're rational, they have somehow missed this information. If they're just playing mind games, they need to be called out (not your job).
Just my suggestion. Assuming people are rational and just lacking information often gets surprising results, and builds trust over time.