Recursively mosaic rasters by directory (year) in R

Question

I have hundreds of rasters spanning 10 years, files are stored by year (e.g., 2020, 2019, 2018, etc.). I would like to mosaic all the raster files within a directory, move to the next directory and repeat.

Normally, I do something like this to mosaic a list of rasters within a single directory:

(file_list <- list.files(here("file/path"), 
                         pattern="*6.tif$", full.names = TRUE, recursive=FALSE))
# raster list
(rast.list <- lapply(1:length(file_list), function(x) {raster(file_list[x])}))
# And then use do.call on the list of raster objects
rast.list$fun <- max
rast.mosaic <- do.call(mosaic,rast.list)

How can I adapt this so that all the files in a single directory are mosaiced to a single file, and then move to the next directory? I'm using raster, but am open to stars or terra options