Thank-you for your question.
In women aged over 45, menopause and perimenopause should be diagnosed by symptoms, not by blood tests. So with Mirena which presumably is controlling bleeding, you will not know if your periods have stopped or not, but you can still try HRT.
The main part of HRT is estrogen, since when our ovaries work less well, we become gradually low in estrogen and low and changing level of estrogen is likely to be the cause of these symptoms. If we only took estrogen for the HRT, it could stimulate the womb lining and so we also take progestogen to protect the womb lining. With a Mirena in place, it will release progestogen directly into the womb, protecting the womb lining for 5 years. So, you do not need it replaced before starting HRT and you can take estrogen only either as a daily tablet, a daily gel, or a twice weekly or weekly patch. Mirena is licensed for the progestogen part of HRT for 4 years but it is nationally recommended that it can be used for this purpose for 5 years.
Therefore would seem reasonable to have a trial of estrogen only HRT. If estrogen plus Mirena suits you well and you wish to continue this combination, the Mirena would need to be replaced in 2018.
It is unlikely that the mirena is causing these symptoms, more likely is the changing ovarian function and changing estrogen production.
Finally, sensitivity to thrush can be affected by hormone changes and estrogen replacement, particularly vaginal estrogen, may help by restoring vaginal acidity and hence barrier to infection. There are also some acid restoring vaginal gels which may help.
I hope that this is useful