I know it doesn’t meet a lot of your criteria, but I use a set of speakers with an MP3 of a siren.
Lots of apps etc out there that will play this at various volume levels.
You can easily buy a strobe and connect it to an outlet to turn on/off with HE.
As all of my HE gear is on a large UPS, no need for battery backup.
Not an all in one device, but this works for me.
Andy
The aeotec gen 5 doorbell can do most of what you’re asking for. It can be triggered by the button that comes with it, or by any automation because it also has a switch capability.
It’s not as loud as the siren, it has a configurable volume setting from 1-10, there’s even a method to customize the mp3 files that are on the device. There’s a light on the device that does provide a visual indicator of the doorbell having been activated, though I wouldn’t refer to it as a “strobe” since it’s not very bright, just an LED light ring really.
It doesn’t have a battery backup, and it’s not the best option for a siren because it’s not loud enough for that, so I also use an aeotec gen 5 siren for that reason; personally I find the lowest volume setting of 88 dB to be acceptable.
I’m not sure how easily you can find the doorbell for sale these days. At some point aeotec will release their gen 6 combined siren/doorbell I guess, but hard to say when that will be.
Do you use Google Home devices? It's very easy to play an MP3 through your GH devices as a notification. Can even set up quiet hours within the rule if you want.
Thanks everyone, I'm looking into that Aeotec doorbell.
Ryan, I do use Google Home, and figured out how to have it announce through Hubitat if any of my leak detectors ping. So thanks for that recommendation.
It would be cool if there was a test button to see how it sounds. But that's another issue.
Got an email saying the Gen6 will be released "end of April for US, end of June for EU."
I would take a look at Cast-web that @bptworld ported to HE.
I haven't gotten around to trying it myself yet, but @Ryan780 was explaining what is possible and it sounds great. Best of all, he said it doesn't deliver that tone when it has to reconnect every 5 minutes which most cast-ready software can't turn off on a Google Home device.
No no no... All credit goes to @jp0550 for that one! 
Not sure what stage it's at now, haven't used it in ages. Using the built in Chromecast app.
Oh, so sorry @jp0550. Saw you asking for help porting Bryan, and didn't read down further. Apologies for misdirecting the credit.
I have the Chromecast Integration working, but I cast to a Google Home Mini and I really dislike the startup tone after it disconnects 5 minutes after idle. On other cast enabled devices, you can turn off the tone, but on Google Home device, you normally cannot.
Just to clarify...i don't know that it doesn't play the startup noise again. I meant it doesn't disconnect as in doesn't update volume or whatnot anymore. I use many of my GH to play music or podcasts or whatnot so i use the broadcast feature for notifications. That way, it keeps going once they are done. That is crucial, especially for the one in the bathroom. 
I see, but it definitely doesn't announce "broadcast from [your username] like Google Assistant Relay started to do. That is a huge advantage, if it can broadcast without adding that like Assistant Relay used to.
That's not assistant relay doing that, that's Google Assistant. And no, it cannot be turned off.
I understand. I meant when I use the Google Assistant Relay driver to access Google Assistant. It was a ridiculously Google thing to do. So if the Cast-Web might makes the same sound, then I won't bother, and will instead continue to use the Chromecast Integration. Either way, won't be able to try it for a while.
The Google Assistant Relay Driver access Google Assistant Relay. LMAO
That is incorrect. Anything that uses the Broadcast capability will broadcast this. Cast web can do either. If you notify the cast-web-api device it uses broadcast to all GH devices. If you "notify" an individual GH device (or group) it uses that cast feature to send the TTS to the speaker, the same way that Hubitat does. So, you have the best of both worlds.
Man, this convo lit up whilst I was away!!
For what it's worth, I just recieved a peice of hardware which might be the answer to this and a many more solutions. Have you guys ever done any work with a Cloudio GraspIO module for the Raspberry Pi kits? The name of the thing will be off-putting to many here, as they would prefer it be a "LocalIO", but for now it's pretty cool.
Essentially mount this little box on a generic Raspberry Pi board, and you can program it to do pretty much ANYTHING you want. I'm not as smart as I should be on the whole Raspberry Pi thing, but I am blown away with the range of capabilities one gets in all computing aspects. That said, it's a very natural fit for the home automation crowd.
LOL... Do a search for Raspberry Pi Hats... Just be prepared to go down a deep rabbit hole as there are probably hundreds of different hats for RPis. 
I'm confused why you think this device will be the end all be all of HA.
I think less of an "end all, be all", and more of a capabilities gap filler. I've always found that if a person or product specializes in one specific function, the result is a far higher quality end product than an All In One (AIO) product solution.
This won't take the place of those devices that are purpose built for a specific function and which do a near flawless job even executing those functions. It will however, being as customizable as it is, be able to bridge the gaps between multiple specialized devices which are speaking slightly different languages, as well as be capable of providing a solution where there is not yet a device for a specific capability.
Ideally, everything would work seamlessly together based on a universal standard and configuration, but as we all can see, this is not the case in home automation yet, not by a long shot.
@pete3500 Have you taken a look at my HubDuino project? It allows very easy integration of microcontrollers like the ESP8266, ESP32, and various Arduino boards. It basically fills the same 'gaps' that you mention above, while maintaining all local processing. Just an idea for you to ponder. Also, the cost is a fraction of what a RPi + Hat would be.
I agree. I would never use a RPi for something an ESP8266 can do.... But that's me.
I have not, but will for sure. I'm pretty sure the part of my brain where languages and math get processed was partially starved of oxygen at some point in my life. The punchline being that I suck at coding... Like REALLY suck, and have to rely on folks like you who are both smart and generous. Thanks for the link!!
I've not screwed around with anything other than RPi, but it sounds like it deserves some study on my part.
Download the Hubitat app
