Instable macbookair after Catalina upgrade

Hi there,

not an easy question, I read a lot and went to Genius bar as well, please help!

My macbook air is from 2012. it was perfectly working before upgrading to Catalina.

Now, it is impossible to use: spinning wheel. switching off. cannot restart witout internet recovery..

So I called the support to reinstall Catalina. it did not fix.

Sometimes, in Disk Utility I cannot see the internal drive "Macintosh HD". it's random when I can see it or not (with appropriate filter of course)

I went to Genius Bar, the Genius guy plugged the power + internet cables. Disk Utility was showing the internal drive (which I could not see before). full install of Catalina - factory reset.

I imported data from backup including apps.

few days later same instability, crashing etc

I did again full install without importing data.

it's now even worse.

I thought it was due to Google Chrome (I saw some people having issues with Google Drive)

but even with safe mode start the computer is slow and finishes by switching off..

I installed a software to scan the disk: SMART Utility | Volitans Software

the result is OK no errors etc

question is whether I should downgrade to High Sierra?

I'm surprised to see all these people having same issue but could not fix it.

Your help is much appreciated!

Should you need more info please let me know.

Best