Welcome to the hobby. I started a year ago with a Nexstar after 30 plus hours on the sim. I used the sim to stop the dumb thumb issue.
1. My OS .46 on my Nexstar was running rich out of the box. Have someone look at it for you. My low end needle was way to rich. Sloppy transitions and problems with engine load up at low throttle. Had a club guy look at it and he walked me through tuning so all is now well.
2. I initially flew with just the wing-tip extensions and I had the same problem your describing with the climbing issue. My plane would pitch nose up in a turn and I would have to use the rudder to kick the nose back over. I removed the wing-tip extensions, and all is now well UNLESS I am turing away from a side wind. Not as pronounced as before, but still there. Just a little rudder to compensate and she flies straight again.
3. The plane is tough. I cartwheeled her twice. No damage. Ran into my rental car while on a business trip, and the car took worse damage. Hard landings? No sweat. The heavy mains take alot of abuse and shrug it off.
4. The only thing I can tell ya about mods is be careful of the connection system for the main gear. Most folks have no problems, but I was one who had to perform a single gear landing cause one of mine got chucked in flight. I was so mad I epoxied the new ones in. A little less anger and I may have thought of using nylon bolts. But it worked. Just keep an eye on yours and always pull out on the gear once you have them attached.
5. Folks may beat you up about having a Nexstar but I firmly believe the plane kept me in the hobbly. I did 30 hours on the sim and sloloed on my second flight and I am no young guy at 43. I fly the thing to death. Thank goodness the thing is durable.

So, welcome again, and the guts of that plane work great in WM T-34 or a WM Sky Raider Mach II.