My imac is totally frozen after the latest system security upgrade (10/2020).

If you have tried booting in Safe Mode without success and erased the drive then reinstalled macOS without success, then you have a major hardware problem.

Make an appointment at the Apple Genius Bar for service after the stores re-open. If you need to find an Apple Store - Find a Store - Apple. Also, see How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD). You may also get Apple authorized service at a local Best Buy or contact Apple Support for more options.

Contacting Apple Customer and Support Service

1. Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
2. To arrange a return, repair, or replacement: Returns  &  Refunds  -  Apple  Store  (U.S.,) and Mac Repair.
3. If you just purchased the product, then you get 90 days of free telephone support from the date of purchase of your Apple product.
4. Primary support help at Contact Support.
5. To contact product and tech support: Contacting Apple for support and service including international calling numbers.
6. Product  Service  and  Repair  information.
7. Apple  Service  Programs.