If you’re doing something and it’s not working, stop doing it!
There’s merit in everything thrown up so far and this is just another suggestion to consider.
I’ve just finished doing Dogg Crapp training for about two months. Strength on target lifts went up nicely by size didn’t seem to follow.
I’ve now switched back to a volume approach and I find I can go heavier than previously. So, so far so good and it will be interesting to see if we can get some more size now.
My suggestion is to try Dogg Crapp style training for biceps only and see what happens. Pick three bicep exercises, do one each workout and workout biceps every second workout. Rep to failure, pause 15s, repeat, pause 15s and then stretch the biceps. I like to pick weights for the exercises that give a variety in rep range (6-12).
Although this may not give you the size you want right away it should give you nice strenght increases that will build the foundation for size.