I recommend asking the GP to try the pill and/ or Mirena coil, which can be amazing with heavy bleeding and pain. You might have to try different pills to find the best one. The Mirena and pill may make the uterus lining thinner, resulting in less bleeding and fewer cramps. It's worth using it as you wait for your surgery and afterwards to slow down the re-growth of lesions, as menopause is not a cure for endometriosis either
If you do get it inserted, take paracetamol and ibuprofen 1h before, and make sure they have lidocaine available to inject into your cervix, and numbing gel to make it as comfortable as possible.
Ask your GP for mefenamic acid and/or naproxen (NSAIDs, which help a lot as they are anti-inflammatories and give pain relief). You can take these 3/4 days before your period and during it, as well as daily or PRN. If you take them with omeprazole or other PPIs to protect your stomach, that should mean it reduces the impact of your IBS.
You can also try tranexamic acid and/or norethisterone (helps reduce heavy bleeding) - you need to start taking it 1/2 days before your period starts so that it can work at its best capacity.
I would also look at other opioids as well, for example, tramadol, or I know people who have oxycodone on PRN at like a 5mg dose. They tend to be stronger and can help a lot more than Co-Codamol. Also, these may have fewer GI/constipation side effects, as they are all different for different people. Personally, with stronger opioids, I have fewer GI side effects, and I do have gastro conditions.
The stages of endometriosis are actually about how it impacts your fertility - so a higher stage has more impact on fertility but not pain. So a person with stage 1 endo could be disabled by it and have severe complications, but a person with stage 4 may not.
Also, if I can ask, where exactly was endometriosis tissue found (which organs) on the MRI, as you may want to discuss surgical options depending on where, and how much was found and exactly where.
If, for example, lots were on the bladder and bowels to make sure sufficient planning is in place for the surgery. Depending on the location, it may affect how much can be excised, from what locations and whether general surg (possibly colorectal sub-speciality) and/or urology, or other surgical specialities need involvement. Otherwise, they may have to remove what they can - then plan another surgery (so you would be waiting longer), where those specialities are.
Also, what medications do you currently take? As for endometriosis and adenomyosis, and for anything else? I know you said you've tried other meds (zoladex and ryeqo). Are you still on them?
What other symptoms do you have? Bowel issues (diarrhoea, constipation, etc?), bladder issues (e.g., urgency, pain or burning), do you have nausea and/or vomiting, back issues, pain with sex or anything else? How severe would you say each of them is?
Do you still have issues with your iron?
The links below have much more detailed and useful information on a variety of treatment options in lots of detail on pros and cons:
• https://www.leedsth.nhs.uk/patients/resources/endometriosis-2/
• https://www.nhs.uk/conditions/adenomyosis/