Xiaomi (including Aqara) devices are about as easy to pair to Hubitat as they are to SmartThings. They use a custom ZigBee profile, so, as you know, it's a bit difficult to pair, but if you get them to pair and stay paired for a while, from what I've read, most people find them to work well (myself included). You'll also need to install a custom driver, just like you had to install a custom DTH in ST. As with ST, you'll need to be careful with ZigBee repeaters, since not all play well with Xiaomi devices. Many helpful forum members have documented their findings here: Xiaomi devices - are they pairing / staying connected for you? (at this point, while it's a bit more difficult than an off-the-shelf solution, if you need a repeater, an Xbee seems to be the way to go).
For notifications, they do plan to release a mobile app with support for those at some point. For right now, you'll have to use a workaround. One is SMS--Hubitat can send up to 10 text messages per day. Another is a third-party notification service, like Pushover, which Hubitat supports. You can also use this community-developed app: Route Hubitat Notifications to SmartThings (App and Driver) (this is what I'm doing at the moment--I still have SmartThings around, though you technically don't even need to keep the hub online for this to work).
Is "KOF" a King of Fans controller, like the Hampton Bay ZigBee Fan controller? If so, many people have problems with those but discover they work better if they are very close to a ZigBee repeater (you will probably need at least one in the same room; I've seen the knowledgeable @JDRoberts recommend two with 90 degrees of separation, i.e., adjacent walls, as well, if you still have problems). I have one Xbee in the same room and it works well for me now--it used to "fall off" all the time before I kept a repeater nearby.
For your list of devices, you'll have to check for specific compatibility, but almost any Z-Wave or ZigBee device that works with ST will work with Hubitat, either natively or with a custom driver. If there's a custom device type handler (DTH) for ST and it doesn't work natively with Hubitat, they are usually very easy to port. A list of known compatible devices is here: Compatible Devices Wiki. There is a similar thread for incompatible devices, but again, most that work with ST should work here as well.
For your automations/SmartApps, Hubitat supports apps and has several stock/built-in apps for common tasks (e.g., turning lights on with motion). It also includes a rule engine called Rule Machine that you can configure to do almost anything if a stock or custom app doesn't do it for you. If you're familiar with webCoRE, there is a community-modified version you can also install. I do see you mentioned ActionTiles. They initially indiciated they might be willing to make a Hubitat-compatible version (and Hubitat believes, as do I, that the effort involved would be minimal), but lately it seems they are not interested, so I wouldn't count on that. You have a few options: you can integrate most devices from Hubitat into ST using a community Other Hub Integration app (and then keep using those devices in ST for ActionTiles), or you can give Hubitat Dashboard a try (this is similar in concept and relatively new but natively supported; there are just a couple of features I'd like to see that aren't there yet--and it will also give you options for remote control since there is no mobile app).
As far as pairing devices, you'll have to remove/un-pair the device from ST, then pair it to Hubitat, and you'll have to do them all individually. Unfortunately, this may take a while if you have a lot of devices (you don't technically have to remove all of them from ST if you "reset" them, but I find it's usually easier to do it the "right" way, and then you also don't risk ST re-discovering them before you get them on Hubitat). Not much different from how you'd have to do it on ST or any HA platform. Some people have created intricate methods for how they move devices (starting closer to the hub and moving farther away to expand the mesh reliably, for example--but also potentially making it worse for the hub you're moving away from, i.e., ST, so I'm not sure one way is necessarily better than another).
Hope this is helpful! If you have any questions, I'm sure lots of people would be happy to help.
