I have a zigbew network running with around 55 devices current. Ever since matter over thread I'm wondering whether I should start making the switch.
Tell me, what made you start migrating to Matter? General availability of hardware, latency, stability, something else?
Fast forward some years and I had my share of vendor specific ecosystems be shut down, leaving me with a bunch of technically fine paperweights, which was when I started focusing on local first access. I didn't want to be locked in by yet another vendor, and I was still using HomeKit as my main platform (still not very advanced home automation), so thread/matter came naturally.
What really turned things around was when I purchased a Homey Pro in 2022/2023. I could integrate pretty much everything locally, and expose it to HomeKit to the "control surface" my family used, which is when I started using Zigbee more (only had Hue before that).
These days I'm running Home Assistant. The Homey Pro became too small, frequently running out of memory, and switching to Home Assistant opened a whole new set of doors with ESPHome and others, like gaining control of my Vaillant Heat pump thought an ESP32 [1], or reading the wmBus telegrams from my water meter.
My "main criteria" at the moment is not so much if it's thread, zigbee, or wifi, but heavily focused on local control. I'm only buying devices that have local access. I'm fine with devices offering an optional cloud plan, like my Tado X TRVs, but I must be able to access them locally from HA.
I still default to thread/matter where possible as to not have too many things going at once, and I have zero Z-Wave devices.