I’m sure that was very frightening. Have you checked with the local police department’s community liaison officer to find out why they didn’t respond to your 911 call? That would be my primary concern.
As far as the Siren goes, The next thing is to check with your local township. There are normally pretty restrictive limits on both the siren and the strobe at a residence. Just imagine the firefighters were coming to put out a fire at your home and they were suddenly blinded by your security system. Not good for anybody.
So as far as finding the “loudest“ etc. They are almost all limited by law to a specific maximum level, so they’re all about the same in that regard if they are wired. Battery operated ones are sometimes a little quieter.
Since there is a private security force in your community, have you talked to them to see what they recommend? Many will have a list of specific models.
Also, just to get this out-of-the-way… This forum is for people using the Samsung SmartThings ™ Home automation platform. It’s not a general home automation group. So all of the questions and answers are soon to be within the context of that platform. That would limit you to sirens that work with that platform, and that probably wouldn’t work without it.
This is a very active forum, so it often comes up near the top of general Google searches, but it’s not a general forum.
Are you using the Samsung system? If not, you might want to ask again at the Reddit home automation subforum as they are brand agnostic and you will probably get a more useful answer.