Do we have Writing SE adds out on the web?
Or, if not, do we have any adds on other SE sites?
If not that either, are there plans to?
If not, could we start planning?
If not, I've run out of questions about this.
I can tell about the mechanism for 2.: graduated sites have so-called community-ads:
What are Community Promotion Ads?
Community Promotion Ads are community-vetted advertisements that will show up on the main site, in the right sidebar. Community members provide images of the advertisements in answers to a designated question for the year tagged community-ads, and community voting will enable the advertisements to be shown. The ads are cleared and refreshed every year.
Writing's latest installment can be found here: Community Promotion Ads — 2020. Now of course it doesn't make sense to advertise on our own site; instead, you should look for related sites in the network and post your advertisement there. I wrote a SEDE query to look for them; they usually post a new one every January, but I now see this hasn't happened in 2021 yet. I've notified Stack Exchange staff about this now: Will there be Community Promotion Ads in 2021, if so when?