Analysing State changes in ADO

I am using Power BI to create a dashboard for a project we are running in Azure DevOps.  There are around 3500 work items in various states from To-do through to Done.  When extracted as a monthly analytics view there are around 57,000 rows with some items appearing only once and others up to 30+ times (yes it's been a long project!)  For each row in the table I want to know if the state of the work item has changed since the previous month (or week or day if I change the granularity) and what state it was previously.  From this I want to be able to count the number of times the state has changed, say from 'In Progress' to 'QA Ready' or vice versa and do various other analysis on the state changes such as duration in 'QA Ready'.

I also want to do similar things with change of assignee, but once I have the above this will just be a variant of the same DAX formulae.

I expect this problem has been solved many time and hope that someone has a nice blog or video describing their approach.

Any suggestions?  Thanks for looking.