You need to pay money to get advertorials in big newspapers or niche sites about buddhism. Press releases are cheap but will have no traffic, nobody will find out. Press releases are usually done for SEO (to get backlinks) on relatively cheap sites with high google page authority, only newspaper publishings are done for traffic.
Honestly I have no idea how this should be promoted in a buddhist field. In any case it would be good to have a big article in a big newspaper or a couple of them. In my country such things are cheap, ranging from 150$ to 1000$ in biggest newspaper/tv sites. In USA it might cost more, so I supposed it would be better to pay for niche buddhist sites. Maybe they will even do it for free in case it is a big event.
In order to be a big event, there needs to be a compiled version of the nikayas, otherwise it’s totally useless and invisible in terms of publicity. This needs to be a new nikaya translation, on par with B.Bodhi one, done for free, etc. This is how it should be presented, as something big, not as a simple “B.Sujato just finished completing all the suttas on his website”. You need a prinded book with a cover, even if it’s not gona be sold, it’s important for the image. People will reed it online, but you need a real world book too.
Also, the suttas are half useless for practical proposes if they are not compiled into a book. They are only good for quoting them on forums or searching a particular sutta on a google. A person who wants to read the whole book will need it structured in a particular way, with a table of contents, etc. It is ridiculous to do such a titanic job of translating the nikayas, but not have them assambled into books.
PS: If I were to have such a titanic work finally completed, I would not waste this oportunity that I have to give a big slap to Wisdom Corporation. I would pay a couple of grand to get articles on super important sites, and never forget to mention how this new version of the sutta pitaka is free, not 200$. It would be advertised as “the first free version of sutta pitaka - now all can have access to the dhamma, even those without money” or “first suta pitakka completely free, with no profits intended” - something like that
PS 2: If there is no time at the moment to publish a printed version and launch a decent publicity campaign, then I think it can still be done at a later moment and nothing is lost. They can be published on the site for now, have basically zero publicity, and do a serious campaing a couple of months from now when they will be compiled into a book and print something like 10 versions. And why not even distribute something like 30 printed versions for free just to put the cherry on top of the publicity campaign, despite not having printed versions avaliable for sale. Or if you do have printed versions avaliable for sale, they should be at least half the price of wisdom corporation or even a quarter, done somewhere in myanmar/thailand/china or some poor country close to australia. And of course make sure to stress how “they only cost the production costs and the online version if of course free”