Thanks to all the replies!
Here's a run-down of your suggestions for preferably free tools for making flow charts:
#inkscape
Affinity Designer
Excalidraw.com
apps.diagram.net
mermaid.live
DiagrammeR (https://cran.r-project.org/web/packages/DiagrammeR/index.html)
and good old powerpoint
In the end, I remembered we have visio in our office subscription, so I just used that. It was boring, but got the job done.
I look forward to digging into some of the tools y'all suggested on another project. Thanks for the tips, #FediFriends!