the articles that are already indexed in google should stay on the old site. The non-indexed ones you can move.
However, getting very little traffic from 1400 articles is pretty skewed... something's not right and it can be any of these reasons: you have entered in a very touch niche (medical, banking, web hosting, etc), you have not targeted any keywords, or the wrong ones, your site has been penalized for some reason, your content is not unique, etc.
What I'm trying to say is that if the content is good and you've done your keyword research properly and the niche is approachable you need to run a site audit and see where the problems are coming from because it's a pity giving up so much work. Sometimes, just a few strong backlinks, or fixing some issues you might have with the site, or adding a bit more content to your pages could shoot some of those articles straight to page 1 of google and flood you in traffic and sales.
Seriously, run a site audit (with several tools if you can afford, just for double safety) and see what each tool says is wrong with your site and fix those issues. Alternatively, take a good look at your competition, look at their content, their sites' metrics, and their backlinking profile (only do this for the sites ranking in top 5 of google, no need to bother with all 10 spots) and then compare those results to yours, and also to the results of the site audit and see if it's worth fixing the issues. And if it's not (maybe it takes too much time, or too much money to fix 1400 articles and it might not be resource-efficient doing this) then you can get the articles that are not indexed in google yet (this is very important) and put then on a new domain... or even an expired one, but make sure it's in the absolute exact niche as your current domain and that the content and backlinking profile of the expired domain is safe and clean
