You could ask 10 different SEO's this question and get 10 different answers. 8 of those answers haven't tested anything and are just repeating a version of the first two. Take any advice you get here with a grain of salt.
1) Is it worth it publishing 10-20 articles and let it sit for 6 months and then come back and start publishing more content? In the meantime, I could create 5 more sites and publish 10-20 articles there.
It really depends on
why you want to do this.
Either:
A) You want to build all 5 sites up and you want to start them aging quickly.
B) You're looking for the easy wins and you'll scale the ones which perform well, to begin with.
Whether it's a good idea... kind of depends.
A) I'm currently building 6 SEO projects and I've started them all at the same time for the same reason. If you genuinely want to build up all 5 then I think it's the way to go
if this isn't your first time building niche sites. Humans are awful for optimism bias when it comes to predicting how much energy or time they'll have for a project. Building 5 sites is fricking hard work and it's going to be expensive if you're building links. You might be better focusing on one at a time if your time or budget is limited.
B) Always worth it. I've seen case studies of this in the past and testing niches with small sites, to begin with, is how I've found some of my easiest wins in SEO.
2) The articles that I publish after 6 months (when the site is out of the sandbox), will they rank faster or will it take another 6 months for new articles to rank?
They'll rank faster than the original 10-20 but it'll still not be overnight.
If you're just sitting around on your hands then you might as well keep publishing to those sites. The only advantage to waiting is if you're putting time into the other sites at the same time.
the sandbox period is per post and not per site, so every new post will be in the sandbox for 6-8 months. Once it is out of the sanbox it will start ranking, but there is no guarantee that the article will actually rank
Not the case in my experience. Not even close. Parasite SEO alone should prove each individual page is not in a sandbox for 6-8 months.