Hi and Welcome to the HASS community,
I do apologise if this comes across as being unkind, but looking at the read / reply stats on you post, many folk have run a mile from a large, detailed post.
Start small, use what you have, don't try to do everything at once.
A RPi4 with 2Mb RAM is fine, so 8M will also be fine - you can even use the RPi Foundation Imager tool to directly install HASSOS:
The HASS Yellow is a RPi4 in a box with inbuilt radios, and mine runs HASSOS, HASS, AdGuard, and a load of other stuff just fine (Z-Wave, BTLE, MQTT, Zigbee, Matter, Thread and loads more).
The simplest Zigbee USB radio is probably SkyConnect as it's made by SilLabs and Nabu Casa (who created HASS). There's lots of others though.
Zigbee works, WLAN works, Matter / Tread will work better next year once the hardware is widely available (we've been saying this for 2 years...).
Use this forum as a reference, and to get the best out of the community, read this: