I have 4 Hubitat Hubs interconnected for my home with 165 devices. Much of my automation is done via Node-Red and MakerAPI. Each hub is connected to Node-Red and in that sense there's no master hub. Node-Red simply sees all the traffic from all the hubs for all the devices I've picked for Node-Red's need to know 
I have zero 300 series devices any more, although that wasn't true 2 years ago. Hubitat has a polling mechanism for them but you're probably correct to isolate them onto a single hub. Over time, I hope that you would, like so many of us, reduce the 300 series from your home.
I chose to allocate 3 hubs to areas of my home. Approximately the first third of the home is on a single hub which I label "Front." This is an area that has the highest density of devices, and my goal is to limit a hub to 65 devices. There's a hub for "Upstairs" and another for "Downstairs." Note that "65" is my personal limit with entirely arbitrary reasoning. There are many people here that have 200 devices on a single hub. I Want, Need, Crave, multiple hubs. 
The 4th hub is dedicated to internet facing actions. I have several internet products in use and they all funnel through my "coordinator" hub. Alexa, Google Home, weather, Presence, and so on. Additionally Hubitat's Dashboards are centered there and I send all the hub's traffic to the "coordinator" so that they can be visualized on a single set of 10+ dashboards. It is also the hub that connects to Homebridge and thus to Apple's HomeKit. I use Apple's Home app as a Dashboard too. I must comment that I barely use dashboards in real life, yet I've set up two complete sets, Apple's and Hubitat's. I think it's just historical.. when starting out, the Dashboards were an easy confirmation that the system was working as intended. As time went on, and the system remained stable, the need to constantly look diminished. These days it's mostly a way to discuss having a smart home with Guests.
Since all the devices are 'mirrored' to my "coordinator" and internet services tend to be single hub focused, that's where I connect to both Alexa and Google Home. I have a single Google Home that I use exclusively for announcements. It's mic is disabled and it never does anything but play sounds when needed. I don't remember the count of Echo's but Alexa is willing to turn my lights on or off as commanded. My wife uses it the most, my kids not so much, me, never... except to show off "smart home" to guests. I've never had my wife tell me Alexa is broken, and when showing off to guests, she's always worked. That's not much of a commentary on her reliability because the home is so highly automated that the usual tasks of turning on or off a light is rare. Let me detail the path that Alexi uses... the connection is via "Coordinator hub" which has zero real devices and 165 mirrored virtual devices. Alexa wants to turn on or off a light or fan, and she tells the coordinator which sends that to the hub that does have the Z-Device connection. That hub gets the command, does it and reports status. That status also gets mirrored to coordinator which informs Alexa. In other words a lot of moving parts but I haven't had an Alexa problem in at least 18 months, probably more.
I don't know, and can't say I've heard of, Satel but if it's a wired Alarm panel, then perhaps Konnected will be something to pursue.
I have never looked at Hubitat's Mobile App because I started my Hubitat journey long before it existed and by the time it came out, I didn't (don't) need it.. BUT it's largely just a combination of Presence + Dashboards + Notifications. Thus it's a reasonable product that seems to work.
You've read enough on this forum to know this but let me make sure: Hubitat is Event Driven. Devices connect to Hubitat via Drivers that convert device specific into Events, via Capabilities, Commands and Attributes. A driver, be it for Zwave, Zigbee or an Ethernet (WiFi) device, focuses down to an Event. Up, down, opened, closed, active, inactive, sunrise, sunset, 9pm Tuesday, would be a short list of Events. An Application on Hubitat is where one or more individual Devices are selected by capability. Got a device that reports wind speed? An app would be used to check that Attribute (windSpeed) for a value then perhaps checking doors and windows and respond with a notification.
Rooms are a new construct to Hubitat. They aren't full featured yet... You can certainly put devices into rooms.
I think that's a good description of ZWave (and Zigbee, probably WiFi too) generally. Every day there's a report of something not working for one individual that works for several other individuals. Schlage Locks are an almost perfect example. (I have Yale Locks so I don't have any direct knowledge of Schlage.) People with the exact same model will have different experiences. Could that happen to you, yea, could happen, but quite obviously the Community will help as best they can.
ZWave supports Association but since the Hub is doing the same job, we don't see a lot of Associations. There's a vendor that's created a tool to assist with Associations, but I've never used it.
I think you've received enough data to understand that Node-Red is going to be easy for you with Hubitat. It's been perfect for me.
This is a long enough reply and although I can go on for another hour, I'll spare all of us that horror...
