I have been interested for quite a while in running a cluster computer using Rapsberry Pis and running software like Spark. Unfortunately, I have been too busy to work on this much. But when I do find the time, I would probably review the material in this blog post by Ashley Whittaker. He offers a nice overview of how things have changed and made things a bit easier, but is also honest about the problems that remain.
Ashley Whittaker. Five years of Raspberry Pi clusters. Raspberry Pi Blog 2020-04-07. Available in html format.