Update: Gingerbread FTW
I took my phone to the nearest T-mobile retail shop to have them install the upgrade but they didn't have it. The guy suggested just doing a warranty replacement next-day since the new unit should come with the OS update, so I did (cost me 5 bucks for shipping, i have the insurance plan for replacing lost/damaged phone). New unit arrived, swapped my battery, SIM card, and flash card into it. It's working WAY better.
The most important improvement: Battery now lasts about 3x longer (same physical battery as before). I can actually make it through an entire day and even overnight without having to plug in. They obviously cleaned up some power hogs in the OS cycles.
Generally seems more stable and responsive after about a week of use, happy with the phone again. Although, something that seems to be missing is I can no longer seem to find where to direct the primary storage (for apps, photos, etc.) to be the flash card instead of the phone memory, like I did before.
Of course, some of those other problems didn't show up until weeks/months later so I'll post again if any oddness happens. But in the meantime, I definitely recommend upgrading to the latest OS if you have this device.