Improve progress bars + parallel processing

[mccarthy-m-g]

{purrr} 1.2.0 fixes problems with named progress bars:

in_parallel() now works in conjunction with string and list values supplied to the .progress argument of map functions (#1203).

So we could switch over to this from furrr/future/progressr once this is released. This would allow us to simplify our codebase, improve progress messaging, and get parallel processing improvements (https://www.tidyverse.org/blog/2025/07/purrr-1-1-0-parallel/#production-ready-with-mirai)

[mccarthy-m-g]

purrr 1.2.0 was released, so we can consider implementing this now