Considering the type of games you are doing,
the only real choice would be Unity.
That being said, you would have to redo everything but textures and sounds from the scratch.
Unity is good, because asset shop, tutorials, low cost, C# and the like.
And yes, if you load data from xml files for example, it can be made modder-friendly (there is even a way to extend code via dll, look at RimWorld)
But apart from that, maybe consider working on Unity and seeing how it goes as a side project to your games. An effort may not be worth all the work you will put in.
Having an already established code base (btw, give us some snippets of your old code, so we could all have a laugh - 2004 was when Clean Code wasn't popular yet, right?) - there is no reason to change.
I mean, SpiderWeb does it's crappy games reusing everything over and over, and they are still on the market, so why bother?
Improve upon an existing engine, but more importantly - improve game design.
I own enough of your games to know that design and gameplay is something you don't do well.
There is this book "A theory of fun for game design" - an indepth read is in order.
I won't elaborate on this, unless you want me to - just know, that graphics will always be secondary to gameplay fun that peps will have when playing your games. But lack of fun is a cardinal sin.
I know you are user friendly Steven, I can see you try, but every time I look at your games, I see features that were not quite balanced, missing something, or not quite well implemented.
I know you can do better