Your post covers a lot of ground so I'll just post some random thoughts.
That mill appears to be a variant (near copy) of the Sieg X2 mini-mill ? If I'm wrong about that the rest of this post won't make much sense
A search for member Hoss2006 will find much info on converting these mills including a mega-thread. His website is mostly dead links but it's archived near complete.
Link --> https://web.archive.org/web/20161014...onversion.html

Automationtechnoligiesinc has a bolt-on conversion kit but I suspect you're trying to avoid that type of expense. It really is kind of pricey.

I have C7 ball screws on my mini-mill and they're still fine after many years of occasional use. The large flanged type ball nuts commonly listed for sale are too big to fit under a mini-mill table. Or better said they're too large for the style of conversion I've done, maybe you have something else in mind. Chinese suppliers will machine a ball screw to your specs but they all seem to be selling the same large flange ball nuts.

Manual machining with ball screws is doable but not the best. The table locks help here.

I use Geckos but there's better bang for your buck available made in China. Hopefully others weigh in on that topic.

Mach3 is nearly abandonware, go with something in active development like uccnc or mach4 or linuxcnc.
Fusion 360 is great if you're willing to put in the time to learn. If you don't trust Autodesk (and I don't) FreeCad is another option but honestly Fusion is easier to learn.