First time HA user. Switching from Smartthings and Alexa

[feral locust]

Hi. I have a few smart devices such as light switches, smart plugs, led controllers, bulbs, smart locks, cameras, energy monitors, doorbells, smart appliances, pool controllers etc, these use wifi/zwave/zigbee. I was using Smartthings and Alexa, or the manufacturers apps to control these, but I am looking to integrate everything and have it all in one interface. I was thinking of switching over to HA for reliability, integrations and open access.

How well does it work with Alexa?
Would I have any use of Smartthings hub or should I ditch it?

I have a spare Raspberry Pi 4 model B. I also have a dedicated PC for Blue Iris (cameras). I'm not sure which one would be better for HA. Is Rpi4 enough?
I assume I would need a Z-wave and Zigbee usb dongle. Can you recommend some affordable option?

[lime phoenix]

A Pi 4 will probably work.. but i'd recommend getting a sub-$100 mini PC off eBay for stuff like Home Assistant. It'll vastly outperform the Pi and give you room to expand in the future your homelab services.

[lime phoenix]

I use HA with Alexa and it works fine. Nabu Casa subscription makes it easy but it's possible without as well (which I use.) You can use the SmartThings hub for ZW/ZB but I'd recommend grabbing a USB/Networked hub for both instead. It'll give you local control and speed up your network.

I use the Zooz Z-Wave stick, I have the older version though. Here's the newer one: https://www.thesmartesthouse.com/collections/zooz/products/zooz-800-series-z-wave-long-range-usb-stick-zst39

As for Zigbee, the Sonoff dongle seems highly recommended with Zigbee2MQTT.

Make sure if you go the USB route you use a USB extension cable to avoid interference.