So are you saying you are currently progressing again on bench or have you still stalled?
It sounds like you are doing things right in that you are swapping out a lift when it stalls and/or altering your rep range, which is exactly what you should be doing. I like your thinking there. But are you saying when you come back to the original lift, you aren't able to push past your previous best?
Have you only just added the increased oats and protein?
If you find the lower body day to be effective but are stalling on the upper body day, just keep the lower body as-is and change the upper body lifts. It sounds like you're doing this already though, which is good news
If you've been using the How to Grow template for the past year, it's possible that you could now just be stale on it and you may be better served to try out a dual factor approach. Particularly as you're now more experienced and have presumably built up more of a strength base than where you were a year ago. Something worth considering anyway