Yea.. I came from the ADT Smartthings hub as well. With Node Red I have relegated Smartthings to a hubless configuration just used to feed my cameras events in to hubitat. This is simply because Hubitat doesn't support cameras, but Smartthngs does.
Node-Red really feels like a aggregator for HA systems. My writeup at the link I provided was about setting it all up on a Raspberry PI so it should not be to costly to just try out that method to see if it is for you. I run it on a Unraid server, but that was hardware i already had.
Simply put using the Samsung Automation Studio with Node-Red will expose the device characteristics allowed in Smartthings to do whatever you want. You can also pass commands back to Smartthings so that is why you can trigger your remote devices connected to Smartthings to say flash lights, or trigger sirens based on events in HE. It will require a bit of setup, but once it is done you should be able to leave it alone except for the occasional software update.
In my setup I have Node-Red setup to have a restful-api endpoint available for each arlo Camera. It receives Motion events from Smartthing when they happen. Then Node red also has a flow that polls all the cameras periodically for changes of various attributes. These are on/off state, Light state, Siren State, ect. Node-Red will then send the update to HE if those various states change. Lastly Node Red with the HE pallet monitors the devices in HE and will update them in Smartthings if they change. This also applies to the lights and the siren state. My wife accidently tested it out a few weeks ago. Both my Arlo Pro3 camera and my Arlo video doorbell had their siren going off, and then also deactivated when HSM was disarmed. So that seems to match what you are saying.
The limitation of this method is that it is heavily dependent on what Smartthings supports since you are essentially piggy backing off of that for HE. So if it isn't in Smartthings it wouldn't be possible with this method.
Unfortunately cameras in general are a fairly big ask and well have no sign of getting much better support. I think the problem is that since most of them are so cloud based which is not the best for home Automation it really isn't on the offical radar to be improved. For cameras that support RSTP though many of those platforms that record and support it will call a webhook such as what TinyCam Connector Supports so that is a option for them.
As far as outdoor motion sensor, and lights i will assume it is only outdoor motion sensors you are questing are the Ring camera ones. The rest are already attached to Hubitat. If that is the case then simply using a restful api endpoint as described above for motion events for the camera will fill this needed. Once it gets to node red and it passes the event to the virtual device in HE then your automations would trigger off that.
The problem with this is that obviously we would like everything local in HE, but if it isn't this may be the best of the not optimal options.