iMac OS issue for 27inch 2020 model

This computer qualifies for every newer version of macOS including the September 16, 2024 release of macOS 15 Sequoia

Depending on the specific hardware configuration ie: ALL SSD drive or the Combo Fusion Drive ( SSD and Rotational drive )

All versions of macOS starting with macOS 11 Big Sur and above

The Operating Systems are designed and geared towards for SSD only drives for best performances.

macOS Sequoia compatible Computers 

macOS Sonoma is compatible with these computers

Computer Qualifying for macOS 13 Ventura 

Computer Qualify for the Monterey upgrade

macOS Big Sur 11 hardware requirements

Always make a Time Machine Backup  before doing any upDates / upGrades 

You want to review your third-party applications and vendor printer/scanner drivers (unless using Apple's AirPrint) for newer operating system version support requirements. That may be informative about "the best" newer operating system you choose.