Aurora R8, CPU upgrade from i5-9400f?

Is the power source 460W or 850W?

The highest CPU range can be the i9900K socket Intel 1151, you can't install anything newer there! 10g intel and 11g intel is socket 1200.

Board replacement is possible. Buy a new Aurora R12 board on ebay for $199. Then it is possible to install an Inel i9 11900K.

Recalculate the costs of the upgrade and then the purchase of a new PC. What is the best choice for you?

It would be very difficult to change motherboard to a newer Aurora. R8 power switch is embedded in proprietary multi pin led controller cable and power switch pinout varies from model to model.  In addition, R8 is the last model in the gen2 chassis. R9-12 use gen 3 pod like chassis which means led design is completely different. Even if you manage to turn on a R11/12 board installed in R8 case, it is virtually impossible to turn on R8 case led. If you want to keep the case led, that is a serious down side. 
Sell the entire system and buy a newer model may be the only option, which is what Dell wants old customers to do.

Where would you recommend selling my current PC?

I am not sure where but I know this forum does not let user list pc for sale.  I have seen people sell aurora on eBay.