as little coding as possible

HA isn't for you. While you can do most things via the GUI, you need to have pretty deep knowledge of Linux processes (cron, folder structures, configs, etc.), Python, MQTT, in order to take full advantage of HA's capabilities.

However, if all you want is to control some lights, maybe a motion sensor here and there, then the GUI tools are probably enough.

must retain Google home and Alexa integration

You can do whatever you want in HA and it won't affect Google/Amazon integrations. If anything, HA will enhance your experience. For example, set up automations on devices across vendors that don't normally have direct connections with each other, e.g. detect motion on your Samsung device then send a notification to Alexa.

ability to control devices if my internet goes out

Depends. If the integration supports pure local push/local polls, then yes, you can. Certain other integrations require Cloud connections (e.g. Google Nest, Ecobee, etc.)

ability to set device state after powered on

You can do this via HA, yes.

a mobile interface / app that doesn't lag or hang

Yes, HA app is fantastic.

report the incorrect bulb brightness

Depends on the vendor. For example, I've found that my TP-Link KASA switches regularly reports incorrect states and it takes exactly 30 seconds to poll for the correct states. It's a well-known issue.

the ability to control zigbee devices so I can be fully rid of the smartthings hub

You'll need to purchase a dongle. https://www.amazon.com/GoControl-CECOMINOD016164-HUSBZB-1-USB-Hub/dp/B01GJ826F8