I’ve had my watch over 180 days, trends still not working

It could be that some critical health data, like your DOB, is missing.

• Open the Health app on your iPhone.
• Click on the icon of yourself, the image at the top.
• Click on Health Details.
• Look at the Date of Birth field. If blank, fill that in.
• Open the Fitness app on your iPhone.
• Click on Trends to see if they're back.

And while you are at it, you may also want to verify that the Medical ID section is correct. That is basically a different block of data too.

Apple quote;

How trends are measured

Trends are based on how your metrics have moved over the last 90 days as compared to the last 365 days.

Track your trends in the Activity app – Apple Support