All others coins named "anonymous" are nothing more than a copy of Bitcoin with a mixer.
You can use the mixer directly with Bitcoin : https://bitmixer.io/
this is in my opinion the only really existing niche - all other things can probably be done with bitcoin in the future.
also that sidechains can offer privacy is only half true - you still have to send them to the sidechain and also have to get them out of the chain.
You have to understand that 99% of altcoin are just pump&dump aiming to make some scammer rich.
You don't need any tech for that, just a nice logo some technical words in big letters and that's it.
As you said, sidechain of Bitcoin are dependent on Bitcoin so it cannot be truly anonymous.
If I send X coins on a side chain, I get back X coins. It is the same problem with mixing.