1990 300SE. m103, 722.351. 205,000 miles.

Transmission has always had a slight flare from 2-3, but recently it's gotten a lot worse. It takes awhile to engage reverse, and while waiting for reverse I can hear a "whistling" sound. Also, I need to press on the gas in order for the car to move while it is in drive or reverse (won't move on it's own). Seems like an issue with the torque converter?

Changed the fluid + filter twice (including torque converter. Helped a little. Replaced transmission modulator, adjusted it properly. Again, helped a little with the flare, but it's all still there. Messed around plenty with the bowden cable.

Looks like I'm out of easy fixes. I've heard of the issues with other models, but supposedly it was corrected on my transmission. Want to make sure there is nothing I can do with the transmission still on the car. Looks like I'm probably going to buy a used transmission from a junker, replace whatever internals I can, and then spend some vacation time putting the "new" transmission in. In the meantime, should I just run this transmission into the ground?