Strict Site Isolation should already be enabled in Brave by default as explained here:You can also enable site strict isolation feature if you want to, just search ' isolation '
Site isolation is enabled by default on desktop platforms regardless of how this flag is set; see chrome://process-internals for whether it is currently enabled. Setting this flag to 'Enabled' turns on site isolation regardless of the default. Here, 'Disabled' is a legacy value that actually means 'Default,' in which case site isolation may be already enabled based on platform, enterprise policy, or field trial.