I went on HRT mainly because of hormonal headaches. I also experience aura without traditional migraine, usually a mild headache but it leaves me feeling washed out and I hate it. I have been better since I changed to a conti regime, the changing levels seemed to trigger headaches for me. I do still get the aura migraines now and then and not sure what triggers them, light seems to be a factor. As you have a mirena fitted you should be having stable levels, maybe you need to give it more time. They say at least 3 months and for some people it takes up to 6 months.
Have you investigated other causes? My doctor suggested that neck issues may be aggravating the problems so I had a physio assessment. She recommended exercises and gave me advice on my posture (it was awful!). This seems to have helped though it's hard to separate from thr HRT.