Thanks so much to all of you. I will look into the options. We were selling ad placements direct
All the more reason to keep on doing what you are doing and NOT go to WP, which can lock you into a presentation form subject to more hacks by bad actors than anything else on the web.
Responsive is NOT difficult. It is "different" ... but remains HTML and static (as some of us believe) remains the best of all worlds. There is a place for dynamic ... but that has nothing to do with being RESPONSIVE.
Don't rush into this! You could do more harm, self-inflicted, and expend immense time and effort to end up with less than what you have already.
MEANWHILE, investigate (if you are not already using) editors with GLOBAL search and replace to address your static pages. Insert the necessary CSS and go from there.
If you do go WP, just know there are fixed limits to what that cms can do, and you will FOREVER be forced to maintain it with security updates in the future.
If you are doing MONEY on the web ... WP is "lightweight" in that regard and, personally, I wouldn't want to take two steps backwards to take one step forward.