I would only say 3 out of 4 layers are covered, I'll show you which one isn't protected and the ones which are:
1st layer Firewall: Covered via Window Firewall - also you can use Syshardener settings to add basic Firewall rules to block other points of entry to harden your setup further
2nd layer Backup: This is achieved via Dropbox - maybe add Macrium Reflect too
3rd layer Realtime Protection/BB/Anti-Exe/etc: Windows Defender + Configure Defender - I would also add OSArmor to block script based attacks, you can also whitelist applications that this software may cause a conflict with if any
4th layer Web Browser: this is not protected at all as you stated you have no web extensions, I would recommend adding an adblocker of your choice, ScriptSafe & Emsisoft Browser Security this would stop the following:
Adblocker: Malvertising/Ads
ScriptSafe: unwanted content, iFrames, Script based webpage attacks, Javascript et al
Emsisoft Browser Protection: Phishing, Malware, Trojans, Droppers, Worms etc etc
I hope this helps you going forward.
~LDogg