If you’re going all LED lighting then wattage is unlikely to be an issue. And in a new home you should definitely be going all LED.
Several of us who hang out here have had issues with SmartThings after reaching some number of Z-wave devices. For me, going over 25 or so Z-wave plus switches and dimmers resulted in odd delays and what I interpret as lost commands. Just something to watch for.
If I was starting over, I’d do 90% Zigbee. In particular the new GE/Jasco Zigbee 3.0 line. They’re priced nicely, have significantly smaller housings (helps greatly in multi-gang boxes) and work great. I’ve got 4 of them so far and 3 more purchased but not installed.
Aside from the general Z-wave issues, I’ve been very happy with Zooz ZEN27 dimmers and ZEN26 switches. Zooz was one of the first, I think, to allow 3-way and 4-way installation while using existing dumb switches. In the car of the ZEN26 and ZEN27, you can usually install the smart switch in a 3-way without even rewiring the other side.
And Zooz just this week came out with a new 700-series choosey based series that have extended range and smaller housings.
Last comment: regarding scenes. Don’t. In order to support multi-tapping for scene control you will have to run a custom DTH. Which means you are forced to cloud execution. It’s maddening to tap a switch and have it take a couple of seconds to work. All of the brands of switches and dimmers will work with the standard DTH which executes locally in the hub. So your smart switches react as quickly as dumb switches. Your spouse and children will be much happier. For scenes, create named scenes in the app and trigger them with Alexa or Google.