Wow, your setup sounds a LOT like mine (or at least what it was a year ago).  Except I don’t have any Roku boxes.  I use Kodi instead.
 
I am absolutely on board with your plan at the bottom of your post.
 
SmartThings is my recommended smart home hub of choice.  It has a lot of support for integrations, and it makes management of automation so simple.  I wrote about it on Samsung SmartThings vs Vera – Why I Switched.
 
Logitech Harmony Home Hub is probably one of the most underrated upgrade.  When it first came out, I was very skeptical.  But, what I didn’t realize was how well it would integrate with other home automation systems like SmartThings.  I came to this realization and wrote about it on Hey Google, Turn On the Xbox – Google Home, IFTTT, Harmony Integration.
 
You mentioned wanting media control from the Google Home… I don’t think that would be very useful.  What’s useful is being able to switch Logitech scenes using the Google Home (or Google Assistant on an Android 6.0+ phone).
 
I also really like the Chromecast Audio’s capability to be included in “groups” in the Google Home app so that you can cast music using simultaneous playback.  It’s unfortunate that the standard HDMI Chromecasts don’t support this feature.
 
Regarding Home Automation, here are some tips…
 
Get some GE Z-Wave light switches and swap out some of your old light switches.  Then, you can plug whatever you want into the switched outlet.  The Z-Wave switches are sometimes better than modules that you plug into the outlet, because you then get that convenient switch you can use as a trigger for other things.
 
In SmartThings, you might start creating routines for everything, but it’ll get cluttered and unmanageable very quickly.  Instead, take a look at the SmartApps, and use the Smart Lighting app to add all your rules for lights turning on and off in groups.  I have loaded that one SmartApp with lots of rules, and it keeps the routines page clean.
 
Kwikset 910 is a great choice that I wrote about on DIY Home Security Automation with Z-Wave Deadbolt Lock.  That was from when I had the Vera, but it works even better on SmartThings.  Don’t use the auto-lock physical pin switch on the device, but instead use the SmartThings SmartApp called “Enhanced Auto Lock Door” to have a much more seamless auto-lock when you combine it with a Z-Wave door sensor.  It prevents the door from locking if the door is left open too long.  No more slamming the deadbolt into the door frame, or having the door try locking while in mid-closing!

Oh, cool.  I don’t think the Kwikset 914 was out when I got my 910.  Otherwise, I would have gone for that too.  It is all metal, plus individually numbered keys.
 
With the Harmony Hub, that is cool that you could control the Roku TVs via network, but that’s probably more than what the Harmony Hub is indented to do.  I think of it as a hub to control all the AV devices in one room, because you will need the IR blaster for power control.

Oh, that’s a great question.  Ya, it looks like the action of locking or unlocking is not showing up as a possible trigger of a routine.
 
I wonder if there’s a SmartApp somewhere that would give you that kind of functionality.
 
However, I would probably recommend you not go that route, because if someone breaks into your house buy guessing the passkey or by watching someone enter in the passkey, then you would want the alarm to go off in that case.
 
What you should do is get some SmartThings Arrival Sensors and give them to your cleaning lady and nanny to put on their keychains (or keep in their purse).  You can add those in Add a Thing > Sensors > Presence Sensors > SmartThings Arrival Sensor