I have also Mi 9T and I switched two or so months from weeklies (after following them for more than half a year) to Stable
Reason - weeklies were no more bringing any valuable update to davinci for me (to the contrary, with every new one there was some small glitch here or there). More important - there were no more Security patch updates for davinci, actually Stable has newer Security patch (Jul, 1) than weeklies
You don't need to Format Data
Yes, you will downgrade, but wirhout formatting you will NOT LOOSE any data (I didn't loose absolutely anything, user apps, settings, photos or documents)
But I did the following before flashing the Stable:
- wiped Dalwik and Cache
- wiped /data/system/package_cache
However, you may encounter problems with some apps.
If I recall - it was Galery.
Downgrading MIUI from Weekly 12.5.6 to Stable 12.5.1 caused downgrading the built-in Gallery app but they have dofferent databases format or so, and the downgraded Gallery was crashing for me
Then I simply deleted Cache and Data for Gallery from Settings/Apps, reopened the Gallery and it started to rebuild database - it took a while but after that Gallery works perfectly.
And I did not loose any picture with that (pictures are on Internal memory), Gallery just had to index them and prepare thumbnails for faster browsing through your photos
If you encounter a similar problem with another system app (if it was updated with your recent weeklies), try the same