So I decided that I should ask for help, so here it goes.
Do I want a hub like the Wink or the Smartthings?
Different people, different opinions. Unfortunatly the industry just isn't stable enough to give one or two definitive answers. The closest we could come is your should use Apple, Google or Amazon for voice control ;-)
My preference is Hubitat, but full disclosure.... It's literally still in the mail. It's based only on internet research. I have a SmartThings. I am generally satisfied with performance, but not happy with ongoing support and development. Have not tried Wink, but I've heard it's easier to set up, but has less options, and supports fewer devices.
Is Zigbee, Z Wave or Wifi the way to go?
Yes.
One of my requirements for a central hub is support for both.
Is there a limit to how many devices I sync to these hubs?
Yes.
But what limit for which devices for which hub vary, and often aren't even well defined. For instance, SmartThings only supports 32 end-point Zigbee devices, but not only do some people report having more, but the number can be increased by adding powered repeaters. (Confused? Welcome to Home Automation!)
I like the sounds of a geofence door lock with a combination backup. I hear these run off of battery but what happens when the battery dies?
A true keyless lock should have a backup 9-volt battery terminal. (I went with the Yale B1L, only because the wife wanted real push buttons, for some reason that I didn't question.)
Just FYI, I personally find geofenced door locks for unlocking to be a bad idea. If someone steals your phone, there's a good chance they know where you live. There are many options, including geofence + something else, like a hidden button, RFID, garage door opening, etc. Essentially, a two tier authentication system. (And you can also use WiFi connectively as the "geofence", using a Fing or like product.)
Does it notify you when it is low?
Yes, but many report not getting notice, particularly if using rechargable batteries. See above re: 9-volt; just hide one outside, and try very hard not to forget to test it occasionally.
If my wife and I are home with the doors unlocked and one of us leaves will it trigger the geofence and lock the door or can it be set to only lock if both of us are leaving?
I believe my lock has an option for auto-lock, but with SmartThings/Hubitat/etc., you should be able to set it up however you want.
For instance, my plan is to have motion sensors outside the door (in my case, in the garage). If door is unlocked, and motion sensor triggered (someone must have left), so lock the door after one minute; OR is everyone leaves geofence, lock the doors. Obviously, those sorts of more complex stuff don't need to be set up immediately. (Plus, with the Yale lock, you just push a button, and it locks; no matter what it's hella easier than a key.)
Are there any locks that can also sense when the door is opened and send an alert through Alexa that a door opened and identify which? I would like to use this as a safety measure to alert me if my toddler tries to get out of the house as we now have a pool.
Almost. I believe push notifications are a built in option for the Yale I have, but could certainly be done with SmartThings.
However, not sure on the Alexa notification. It can't be done with Google Home without a man-in-the-middle (such as an always-on phone).
If I were to get a Ring brand door bell camera and an Arlo camera would I be able to view them simultaneously? Would I be able to save their feeds to the same location? Can the motion detection alert me if for instance if it detects large animals such as dogs, coyote, foxes, bear or people without it sensing smaller animals like birds or squirrels?
I have no experience on those, but.... yes, yes and yes. Note that size limit for motion detection would be a limitation/feature of the motion sensor, and not a setting.