Learning new topic: read literature from oldest to newest or the other way around?
Neither.

First, identify the "important" subset of your 400+ papers and read those. Second, try to identify the "good" subset of the papers and read those. Then, (if at all) read the other papers; perhaps you are doing all this with the intention of writing a review, in which case it makes sense to read oldest-to-newest to get a sense of the history.

The most obvious metrics which you can subsitute for "importance" are number of citations (these will likely include review articles) and the ranking (pick one) of the journal of publication. Note that "important" does not mean "good", "honest", "well-written", "reproducible", etc.

For a second sweep, focus more on what you actually do consider to be "good", based on authors and journals that you have found to be of consistent quality.