Excellent report! I’m excited to read your progress.
Try two modifications:
Next time when you’re at a comfortable stretch, alternate clenching the plug as hard as you can for 5 seconds, and completely relaxing for 10 seconds.
Instead of sitting on it, use the plug while on all fours. Attach the plug to a table leg or something similar, and slowly rock yourself back and forth onto it. In that position it’ll be a little easier to relax all the muscles around your ass.