I've managed to get my aging Insteon network to play nicely with my new Thread network, Home Kit and Alexa with the help of Home Assistant. I concur with the above comments that Matter is still in an immature state, but for those early adopters, here's what I did....
Set up Home Assistant on a RPi and connected to my EISY
Connected EISY to Alexa initially via the UD portal
Connected EISY to Apple Home Kit with the bridge add-on
Purchased the Sky Connect dongle and attached to the HA RPi
Configured HA as a Thread Border Router (required some trial and effort using Silicon Labs Multiprotocol add-on)
Used the Alexa app to install an inexpensive Onvi smart plug into the new HA thread mesh network
Shared the smart plug with Apple Home and Alexa and confirmed everything works
Installed Matter Bridge on the HA and added to the HA thread network
Setup Matter Bridge to expose my UDI devices via HA as Matter devices
I shared the Matter Bridge as an aggregator with Alexa
Alexa started to pickup my UDI devices
Disconnected the ISY V3 Alexa skill and switched over to the matter versions of my Insteon lights
Some observations...
Setting up a thread network on HA was a PITA, required lots of trial and error. I managed to get it to work as joint thread and Zigbee using SkyConnect. Others have needed to disable the Zigbee radio.
Matter bridge is pretty cool for a community project. Ideally, I'd prefer UDI to expose my Insteon devices as Matter via the EISY, but that'll take time, per the earlier comments in this forum thread.
Matter is still very generic and makes assumptions about device classifications. Matter bridge classifies my ceiling fans as dimmable lights, which means they don't show up as fans on the Alexa side
I have an Alexa device in pretty much every room of my home, making for plenty of redundant matter controllers. I'm considering picking up some newer Echo 4's so I can have additional TBRs, Presently HA is the only TBR in my home, so when that goes offline so does my matter/thread network
I've not tested it yet, but I should be able to pull my home Internet connection and Alexa devices should still control the Matter and thread devices.
It's an exciting time... hopefully the above helps some other early adopters. In time, many of the major Home Automation players will fully adopt Matter and Thread radios, UDI included. For now, it's all about patience :)