2010 XLE V6, maintenance

I have a 2010 Camry XLE V6 with 148k miles. I bought this car in Washington as the second owner at 90k miles. I’ve since been daily driving it besides the occasional road trip, I drove this car through the Blue mountains in November to Utah and back through 101. Either way I’d appreciate any free advice about what to look for to make the car last, as well as some advice on some issues I’ve been having.

In my time with the car I’ve replaced the water pump, alternator, both front axles, both front control arms, and both front sway bars and the radiator.

The car has recently been drifting left, and then right. Typically when I use the throttle, it will shift left, and then a second later shift right. Where this issue is coming from. This is my main issue currently. What should I look for to diagnose this, and what should I expect to replace in the future.

When I replaced the radiator I had to disconnect the transmission cooler lines, I drained this fluid into a separate container and measured about half a cup. I did not refill the transmission after the repair. I wonder about the condition of the transmission as the fluid from the cooler lines was very black, but did not have an odor. Since then I’ve put at least 3k miles on and haven’t had any issues. Is it worth it to drain and refill, or should I save some money and try to get it rebuilt?

How much should I expect to spend for a rebuilt transmission, and is there anything I should ask for?

I remember a co worker had a Tacoma that had “never been re-geared” and how that cost him gas mileage. Does that apply to this car?

Should I just buy a used transmission and put it in when this one shits itself?