The advantages to paid security are usually addons like backup, multiple devices, lost device recovery, remote data wipe if lost etc.
The advantages of free anti-virus are, free, many have frequent automatic updates, quick response to new threats, scheduled scans at various levels. Good review sites help choice and will refer to these points and disadvantages such as advertising.
It is in my consideration foolish not to extend the Windows Defender coverage but the extent to which you do that depends on some research into the above points, operating ram, memory, amount and intent of browsing and spam received etc..
Another alternative is to go to a Linux OS with basic free antivirus. You save data too that way but it is a slight learning curve.
Backups of data, docs and pics are also certainly as important. Do not rely on one method. I am a retired electronics tech. and have managed a computer store. I have seen quite rare, but devastating virus and other attack destruction. 'Set and forget' of any decently reviewed free antivirus that scans in background and checks realtime should not interfere with computer usage and will, at least once in a lifetime, prove worthwhile for home users in my experience. I use Eset but have used AVG, Kespersky and others, changing according to reviews where pertinent and usually when advertising or purchase prompts get annoying. For anyone with any enterprise where one holds data or information concerning others, the AV is essential.
Malwarebytes free already mentioned is very good software and covers areas not in all AV software but if i recall you do have initiate some scans. I always use it or similar.
Research for an hour, make choice, set and forget, backup all data both on and off site. It is a real world, be careful.