I'm not an expert, but I'm in a similar situation as you, I have to make a choice. So I see our situation like this:
it depends on how big the websites will be, how much you estimate the potential traffic to that site and the monthly earnings. If you are going to launch one big specific project that you believe in, you like website's main topic/niche and you know it will give you a big satisfactory income, and you will drive traffic not only from SEO, but also from social media like instagram, facebook, tiktok, youtube, pinterest. Then devote your time to just this one project. Because working on seo and on social media accounts is absorbing, it takes a lot of time. If you plan to have several small projects (in different niches) and rely only on SEO then I think it is good idea to run several sites simultaneously. You will add articles to each of the websites daily or every few days in a natural way, without rushing. The sites will grow and position themselves for keywords after a few weeks or even months. Remember that if you add too many articles in one day, google may de-index those pages (it may or may not). You have to judge for yourself what kind of plan you have and what kind of results will satisfy you. Good luck.