I am also in a townhouse and can potentially totally save you some bother here.
Firstly, WiFi.. honestly as a previous wired where possible advocate the move to a town house and specifically getting a ring doorbell forced me to sort the WiFi, I have sky q so used their setup which is utter and complete tosh.
I bought one ring and ended up sending it back because it simply didn't work, they are slightly notorious for being twitchy with WiFi hence the chime pro which is a repeater (the cheaper chime is not)..
Anyway when I decided I really needed the camera outside I indulged myself with a bt 3 disc whole home setup and it is staggeringly good. I've basically taken the wires from everything that has WiFi now.. still get a rock solid 45/50mb all over the house on any device and the ring works fine.. the app actually encourages you to move them further apart like a "too easy" chant.
So my view.. pay for that, forget wires. Especially if you're going with a full smart home.
Then voice assistants.. basically they do the same thing, there's little better or best to choose from so it's ecosystem choices again. I personally have wound up in a world with Spotify multiroom via echos. Sonos does have Alexa integration but I read plenty of reports saying the mics aren't great and that is a standout of echos, I find the sound quality perfectly serviceable, it's much cheaper and clearly you want a voice assistant to be able to hear you.
Would I go nest again? Probably not, the nest is the thermostat so placing it in a town house is tricky. As heat rises you have the eternal challenge that if you have it on the bottom floor as we do it can try and bake the top floor etc. We've gotten round it by setting the trvs on the radiators to lessen the impact but given the chance I'd probably go with tado next time round.
I don't believe you can have smart valves with nest.
To be fair the automation, schedules and voice control etc on nest is great I can't fault it.. you can integrate it with IFTTT so for example over winter our heating came on when we started our commutes home. That's not standard functionality which kind of surprises me,you can have it use your phone to tell you've left which is fine but try telling that to a cold house that knows you're home but only when you actually get there which is clearly too late in the depths of winter.
I'm also not aware of any smart product that works better with say Google home than Alexa.. they tend to have feature parity.
Finally, consider smartthings, it's all well and good using your phone or voice to trigger actions but it's even better when they just sort themselves out.
I have motion sensors on each floor that have time based routines for the lights, I use a door sensor on an understairs cupboard so the light is always on when it's open (no other light source obviously) and off when it's closed. One on the front door so the hall light is on when you walk in and some moisture sensors under the sinks for peace of mind.
Then you can start cooking up interesting combinations with it all and smartthings really helps that a lot.
Plenty of fun though!
Oh and robot vacuums... I'd probably give one a go before dropping that much on 3.. bear in mind you'll still need a conventional one anyway.. I'm considering one to scoot around the bottom floor which is all wood but I wouldn't bother with the carpeted rooms, I don't believe they'd be good enough and you'll still need to hoover the stairs or whatever.. it's a LOT of money for something quite niche.
