As well as considering the rest of your post too, I want to start by saying that I do use the ESL40 throughout the day for low intensity long duration stretching but I also do 20 minutes manual stretch under heat as well. So I’m already doing that last bit if your quote. So it’s a bit discouraging that with this combination I’m still stalling right now.
First of all dont get discouraged, you will gain.
A general rule of stretching is that you should not apply high force. Although i think that after the prolonged low tension, your manual stretching should be a little more intense. There is no point to stretch very gently after the low tension-high duration approach. Try to stretch a little more than your Bpfsl if you get what i mean. Fulcrum stretching can be a good solution here.
This proccess can take some time to give gains like all the PE methods. During that time you have to be consistent.
Also dont forget the 5 hours under tension every week with manual stretching. Long duration holds, relaxing into the stretch, no very high forces until the last 10-15 mins. I record the total time every day and when i reach 5 hours or a little more i take 2 days off. Thats what worked for me.