I have started to work on a wrapper around source that saves created variables in the project folder as RDS and optionally read them instead of reexecuting the code, it would come with a naïve progress bar that would move by step or 1/n where n is either the number of calls in the sourced time or the number or variables to read from file. The estimated time would be the last time it was executed.
I have looked around for something similar and didn't find it but it occurred to me I should ask here before moving forward, do you know any package providing such or similar features ?
Do you know upfront the number of files you are going to read (and maybe also their names)? If you do, how about using something like a_ply, that has a built in progress bar (see the .progress argument --- it's a text style one but I find it useful, as it also shows the actual percentage). In the case of a_ply it gets an array, e.g. with the names of files to be loaded, runs a function that you specify on each (e.g. loadRDS), and updates the progress bar on each iteration. That's the simplest "of the top of my head" solution that comes to mind, I may be able to think of other things along this line if that's not sufficient.