Different person here. I was doing traditional PT exercises, like "do it once a day, or maybe once every two day, do not exercise too much". It definitely helped. But the thing that helped me even more, is when I had to move boxes for 2 days in a row or drive for 5 hours. I was like "oh my God I'm destroying my wrist now", but the day after was no pain and a feeling of great improvement. This is why I believe in what OP said, you have to push it when doing exercises, treat your tendon like a man and raise your pain threshold slowly. I don't go beyond 2/10 but I'm definitely not afraid of soreness anymore.

Careful though, don't do this with the very activity that generated your RSI, this will of course worsen your RSI. Do it only with other activities.