mzh140, just to reassure you that your endo hopefully won't come back on HRT - but I totally understand the anxiety, because it was also mine. (Had a lap for endo in 2013 and then was on desogestrel ever since until my early 40s, when I got hit by peri and stopped the desogestrel for HRT.)
Even though I am on an astoundingly high dose of estrogen (300mg Estradot patches plus currently 4 pumps of gel - I am gradually trying to come off the gel) and my estradiol is pretty high around 850pmol, I have ZERO endo symptoms.
Endo is stirred up by fluctuating hormones so make sure you stabilise on a dose, PLUS you will likely need more progesterone than others. (I am now on 300mg from day 5-16 orally and then I add in 200mg Am and 200mg PM vaginally, so total of 700mg/day). I take a break to allow a bleed to happen, otherwise you'll get unpredictable bleeding. So I stop on day 26 and wait for the bleed, which takes about 5 nights to start up - then I give it 4-5 days to come out and start again.
I am currently very very slowly reducing my gel dose to see if I can come off it altogether, as I am bleeding very heavily for 2 days of my period. (Like - flooding to the point I can't leave the house those 2 days.) But I have no pain with it, which is very weird. I'd like to get to around 600pmol and see if that will still be ok for me.
I would probably take a different perspective to others, here, seeing you have already stopped the COC so it makes sense to give the HRT a go. I would suggest you try what I am... You just stop the progesterone to allow a bleed to happen but you take it continuously besides that. And you might need more P than others. If you have trouble getting your GP to prescribe more, you may need to go private. (I am with Newson.) If you still have probs, adding in desogestrel as well (which you can get over the counter and without your GP's prescription, because it is a contraceptive, and it is licensed to be used along with HRT) will help a lot.