Hello - fellow endo-sufferer here, although not severe. I had a lap in 2013 where it was lasered out and then was on desogestrel for about 10 years before hitting peri-menopause and every low estrogen symptom that exists.
So if you have endo, the recommendation is to use continuous progesterone even if you are in peri. And one utrogestan a night (which is the usual continuous amount) might not be enough, you might need 2x utrogestan capsules a night continuously. The Mirena coil is also another option. You can take utrogestan as well as the Mirena if needed, which would give you more progesterone.
Whilst doctors will say that the Mirena by itself is sufficient, I'm sceptical about that. Because it largely works on the uterus and is said to have little to no systemic effects. And of course with endo, we have it outside the uterus and in other places. So I believe we need to have systemic progesterone to counter deposits elsewhere and not just the Mirena or vaginal utrogestan. You can also add in a mini pill with HRT, including desogestrel or your norethisterone, if more progesterone is needed. So basically you have loads of progesterone options but you might need more than other women and you probably need it continuously. To stop breakthrough bleeding, the Mirena or a mini-pill would work, alongside the utrogestan if you like.
I started out on continuous utrogestan but got some breakthrough bleeding because I'm in peri. Whilst my Newson doctor wasn't worried about that, I decided to give sequential a go and see if my endo would be okay with it. So far it has been totally fine, although I only had it mildly in a few spots back in 2013 so perhaps I wasn't a severe case. I am on 2 pumps of gel now with 2x utrogestan from day 14-25 of my cycle and I'm not getting any endo pains. However I need to increase my gel and it remains to be seen if this continues but so far so good.
One theory is that endo is activated not so much by estrogen alone but by hormonal fluctuations. So keeping everything static and stable can suppress it even if you are adding estrogen in with HRT.
It is really worth getting sorted with a menopause clinic like Newson Health I think, for particular situations like this. General GPs have been completely useless for me.