I also wanted this dashboard to be easy to maintain. In my experience, one of the problems I had with dashboards was the long time it demanded to update the data.
I used Mysql to make the queries that would feed my dashboard. When I finished my queries, I began to study ways to optimize them. In doing so, I optimized some queries by almost 90%. I figured out many ways to optimize a query. For example, I discovered that, in some cases, using a left join versus a join could make a huge difference. Also, limiting the query results when necessary helped a lot in the performance.
Then, with a colleague's help, we managed to store this data in a Google Sheet using Appscript. This way, I could access data from every client and keep it in a sheet with just one click. It was a very cheap way of making this happen.
Getting de data
MySQL + Appscript