You would need to host your own or join a matrix server. The benefit of Matrix is you can tie in puppets to things like iMessage, Discord, FB Messenger if you host it yourself. Another option is Beeper (Matrix + all the puppets for other messaging apps) but you still have to sign up and get an invite from them to join for now.
My advice to you will be you should likely look to getting a Developer Account/using a free one from something like AppDB. I am not sure how things will be with sideloading, but services like AppDB have plenty of options sideloading pirated apps. I’m sure that will be updated at some point for wearOS.
The big “but” in all this, is unless it requires you to tie it to an iPhone/Mac, there may not be a way to extract ipa files to sideload.
If you mean WATCHos, then yeah. AppDB sideload to an iphone. Example is like Calcbot 2, you can sideload it to your phone and use the watchos app cause its on your phone.