Not sure how difficult it is to make FUD malware. Some crypter services, whatever they do, they run them through virustotal like services, so several/many/most would be FUD. Also the criminals here are very organized.
Again not sure how long it takes such malware to be detected and added to the database. Considering the prevalance of ransomware it may not be that true me think. But if what you say is true, then windows defender should be the most capable av, and no other av should be required, since its present and enabled in almost every PC.
btw whitestar you sound very knowledgeable in this. I am curious about the source of your knowledge. Any pro experience? NOT trying to diss. Just curious.
Mine is a long time curious nerd, all over the internet.
The prevalence of ransomware is mainly because of "user action" against which there are no solutions(what can you do if somebody decides to click & run a file despite warning) & the fact that most targets use old unpatched windows versions(wannacry exploited smb v1.0 vulnerability for which MS released patch in Apr 2017 & wannacry attack started in may 2017) which cannot be protected by simply using an av.
Windows defender is recently scoring 100% in av-comparatives tests which should tell you something.In fact there is a corporate version of windows defender which is much more advanced.
Microsoft Data Breach & Endpoint Security - Microsoft 365
Why Windows Defender Antivirus is the most deployed in the enterprise - Microsoft Security
Approximately 96% of all malware is polymorphic – meaning that it is only experienced by a single user and device before it is replaced with yet another malware variant.
This is because in most cases malware is caught nearly as fast as it’s created, so malware creators continually evolve to try and stay ahead. Data like this hammer home how important it is to have security solutions in place that are as agile and innovative as the attacks.
I just like to read,nothing else

you can spend some time at wilders forum,best starting point for gathering some good security related technical info(it also has probably the best backup & imaging related forum section with many experts).