In a nutshell, you can learn on pretty much *any* size bike. You may, however, fall-off more often on some bikes than others.
As I said, I believe a 250 c.c. is the ideal size bike for beginners to learn on and this is generally what MSF instructors will also tell you. I’m about the same height/weight as you and my Nighthawk 250 topped-out at 80 m.p.h. on the freeway but going up hills and/or into a headwind slowed it down and required a down-change to get it back up to 65+. Certainly freeway capable but no, not ideal.
The Ninja 250 may be your ideal bike as I believe it can be ridden mildly if you so choose, or, go fast if you rev it higher in each gear (can do ~100 m.p.h. I believe). I don’t know if it will fit you for size though.
Remember, as the graduate of an MSF Basic Rider Course, all you are qualified to do right now, is to ride at less than 18 m.p.h. in an empty parking lot; that’s all. You have a lot to learn about riding a motorcycle in traffic, cornering at speed, hill starts, sharp left and right turns from a stop (junctions) and same turns on up-hill gradients and on down-hill gradients, negotiating poor road surfaces, etc, etc.
This experience is all more easily acquired on a smaller engined, lighter bike. The faster you go, the more danger you are in.
Like I said, you can learn on any bike. As you’ve said yourself that you’ll want another bike in 6 months anyway, why not just bite the bullet and set aside six months as a continuance of your MSF BRC before you “join the club” on the bike you really want. If you buy used, you can sell it in six months for what you paid for it.
Remember that most folks who say a 650 c.c. bike is “a good beginner bike”, have forgotten their shaky mistake-ridden beginnings.
Anyway, I’ve said my piece (at length) and I’m sure you’ll be fine with whatever bike you choose if you take your time and realize that the development of motorcycling skills takes time.