Hi Dan,
the evohome library already implements both v1 and v2 API. It was just the honeywell component still using the v1.
By the way, with this change I also got the error described in https://github.com/home-assistant/home-assistant/issues/8554 . I suspect this is related to API request throttling or Auth token not refreshed, as it begins exactly 1 hour after HA restart.
The guy that answered me from Honeywell is Joe Zwack, I found it commenting on this issue https://github.com/home-assistant/home-assistant/issues/3468 .