Good arch based security/privacy distros? I only know about BlackArch. Are there any good alternatives that are also pretty userfriendly for everyday work/development tasks?

Kali is probably closest to what you want

I've seen something about starting with Manjaro to keep the simplification benefits it gives to Arch and then importing the BlackArch repos... Maybe they will give you what you want?
I usually just used Kali before (especially while Debian was my go-to) but I see BlackArch has so many more tools, definitely gonna give it a look!

I wouldn't call Kali nor BlackArch privacy distributions in the sense of protecting the user. Their goal is pentesting, plain and simple, and come with most of the tools already installed.

If I had to recommend one for security/privacy though, that would be QubesOS. That being said and likes @theKarlisK already mentioned, the best way is to be careful with what you do and install on your machine.