Easiest way is to enable Restore on AC/Power Loss if your bios has that feature and use a wifi plug to do a power cycle whenever you want to start it. I believe this would also works even if the computer is powered off.
All of the instances I looked at have option to block all nsfw contents within your profile settings. I think you can also block specific community/user . But i don't know if there's an option to block a whole instance.