But you are so much more than just an experienced designer. Many construction industries have stand alone full time estimators, a function you are now providing. Most construction industry jobs in larger shops have positions where permit applications and submittal drawings are stand alone positions.
I realize that none of us know your full compensation package and whether or not it includes any kind of healthcare, profit sharing, paid time off, vacation, etc...., but at first glance it sure seems like you are saving the company an enormous amount of money per year.
For the sake of context, I looked up your area's cost of living indexes and how they compare to the U.S. Median averages.
https://www.bestplaces.net/cost_of_living/city/ohio/cincinnati
Your city is a little cheaper than the nationwide average medians in several key areas, but it's not so much lower than the average that you shouldn't be able to command a decent wage. In a couple key areas like Utilities and Transportation costs that aren't as easy to offset, its right at the median.
Your employers should recognize all the assets you're bringing to the table and understand the huge risk they face if you were to leave. If you think it would be impossible to earn much more than you currently are, you have some tough decisions to make.
Good luck. I've been in your exact position several times in my 40 year career and admitting the status quo was untenable and that leaving was always the best move has never not worked out for the best.