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.
If the new reviews are too complex you might need to start with some older ones, but I would really suggest starting from the newest publications. People already spent years and years collecting all the old research, sorting through the mess and writing "concise" reviews about it. There's no need to do the reading to repeat that.
You would need to follow your nose / go down the rabbit hole sometimes, but at least you'd know there's a hole. If you'd start from general/random papers 30-40 years old you'd just read dirt most of the time, research that never went anywhere, or that turned out to be wrong.
Remember that you're studying some scientific field, not the history of the field.
Neither direction works well on its own. I have never profited from a literature review to learn a topic except in this way:
- Skim the Introduction, "Related work" and citation lists of current research. Find sources that introduced new ideas or that constitute the definitive treatment of them, as evidenced by the fact that they keep being cited. Possibly iterate this process once or twice.
- Read those older sources, in temporal order.
- Now read those current texts that seem relevant.
A scientific paper simply lacks the space to properly explain the concepts on which it builds, so inevitably you will have to achieve roughly the same experience and mind-state that the author had to understand it well. Reading what they read is hands-down the simplest way of achieving this.