Big Sur (and newer Macs) come with built in Malware protection. There is no real need to install a third party app. As to viruses, there are no Mac viruses in the wild.
Here are some suggestions for managing privacy and security on your computer:
Level 1 (foundation)
Anti-virus (I use Sophos)
Anti-malware (I use Malwarebytes)
Firefox browser with ad blocking and Javascript blocking add-ons (I use AdBlock Plus and NoScript) for general web browsing. I keep Safari relatively stock and only use it with a very small number of trusted websites.
Level 2 (good to have if you don't mind the convenience vs. security tradeoffs)
Little Snitch (monitor outgoing Internet connections, essentially a reverse firewall)
RansomWhere (anti-ransomware monitor)
SilentKnight (utility for easily checking Apple's own security measures in macOS for updates)
Firefox browser with ad blocking and Javascript blocking add-ons (I use AdBlock Plus and NoScript) for general web browsing. I keep Safari relatively stock and only use it with a very small number of trusted websites.
Level 2 (good to have if you don't mind the convenience vs. security tradeoffs)
Little Snitch (monitor outgoing Internet connections, essentially a reverse firewall)
RansomWhere (anti-ransomware monitor)
SilentKnight (utility for easily checking Apple's own security measures in macOS for updates)
35+ years of using a Mac:
No virus software installed - never got a virus.
No malware software installed - never had malware.
No disk/OS "optimizing/cleaning" software installed - never had issues with my storage disk or the OS.