Good luck trying to send a file compressed with Zstd or Brotli to people and have them know what to do with them. I’d certainly have to google to figure out how to open one of those. And I’m an engineer and aware that those formats exist.
You won't be able to do that with RAR files either. The only archive/compression format that you can send anyone is ZIP, since both Windows and macOS can open them ootb.
I wouldn't send a RAR to my grandparents, but if I had to send pictures for a school reunion I would expect everyone to either know how to open a RAR or to be able to figure it out. Worst case they will google rar and will find WinRAR as the first result.
For zstd or brotli I can't even name a GUI program able to open them outside a somewhat obscure 7zip fork.
Why would you try to compress pictures....in your case zip is as good as everything else (and less problematic to open)
Or if your friends all have WinZip and can tolerate .zipx (which is a file extension for later and thus less interoperable version of ZIP), it does support JPEG recompression so it can be actually better than RAR.
Any lossy compression format it's crap.