Dos is the limiting factor, it only recognizes 640Kb, so just as you have surmised, the software takes a portion of that and you have not a lot left for your programs. This makes you have to do some work arounds..... larger files have to be run in DNC mode, that means you can load it onto the Mill's computer, so you are not relying upon an RS232 connection to drip feed the program from another computer. You also can not edit the program on the mill once you are over the space allotment..... any changes in the program have to be done offline and then fed back to the computer. As for "start from the middle" type of thing, I can't say, never tried it, but the machine will do start from a specific line, or tool normally, so you might be able to do a recovery type situation, I don't know.

I'm not sure at what upgrade point the machine has a larger memory, would have to be if/when they got away from DOS as the operating system. I have a network card and have yet to install it, but it sounds like you have it working correctly. The advantage you will have there is with long programs, feeding them over RS232 can take a long time... I had a 1.5mb program for a 3D part I was doing and the transfer time from my computer to the machine was over 40 minutes.... then if you find an error it's delete it, edit on the pc and then send it again for another 40 minutes. You will get past that with almost instant uploads when networked.

3D paths can be optimized if you run the longest straight directions so the machine has a chance to accelerate. You will need to watch step over amounts as using small ball endmills and really small step overs will make programs huge and machining time long no matter how fast you can get the machine to move. All that said, you will be as fast as anything else out there shy of some $100k machines with high speed processing and specifically set up systems for 3D type work.

You will be best served by good software that can use bull nose end mills to get all the flat areas and then transition to ball end mills for just the areas that you will need them, reducing the time spent profiling with a little tiny contact point of a ball end mill. Sounds like you already have software that's capable.... I'm working with cheap or free stuff and have to cobble programs together to be effective.... pretty hard to justify spending twice what I did on the mill for software to do the occasional 3D part for me.