Magicwriter - after one year of being resident, with private health over , you can pay into the Spanish health system and receive free care as any Spanish national, for 60 euros per person per month
Although existing conditions are often not covered by Spanish health insurance, or at a higher premium, the conditions you mention are common and not too serious, so you might find cover won't be a problem
The medication for hypothyroidism is cheap over the counter
For fibromyalgia I assume you take anti inflammatories, also cheap
You will have to pay a '% of the prescription costs whether under private cover, or Spanish NHS.
No need to worry about the quality of healthcare as it's excellent , but if you are hospitalised and can't care for yourself, your relative will be expected to do all the personal care, which can be a strain if there is only the partner to help, unlike the Spanish who have all their extended family to call on.
Do other research on tax implications of moving to Spain, especially inheritance tax
Also research criteria for becoming a resident.
Make sure you have a financial cushion to fall back on, eg enough money to last for a year.