This is a tough call -- basically, you want to put your money into something that you like, or else you probably won't ride it much after a while. That said, I'm pretty certain your rm could be modified to fit the bill. You may need to resize the ergos of the bike with some taller risers/extenders for handlebar clamps, and taller rise handlebars and seat foam. This, along with the flywheel weight and properly re-done suspension for your weight, should go a long way. It sounds like your jetting in the low and mid range needs to be cleaned up as well, for smoother throttle response and to prevent it from loading up. Along with the needed guards installed on the bike for offroad use, this should be a pretty nice woods bike. With bikes of this age, it seems pretty common that dealers don't stock many parts for them (at least in my area), but at least they can order them, and can also work on your machine if there's something you don't want to work on. Since you've got this bike pretty well ironed out and know its condition well, this is a pretty good route.
But if you're looking to get a new machine, you might consider an enduro machine thats pretty much ready to go, like a ktm250 xc or xc-w (formerly mxc or exc).. Depends how you feel about most likely needing to mail order all needed parts and not having a local mechanic should something major come up. A kdx would also be fine, but it sounds like you're not too excited about it. If you're used to the power of a 250 mx bike, you could be disappointed, but its really in its element on tight, rough-going sections. Just my thoughts, its really a personal decision that you'll need to make after weighing the pros and cons.