>Was Bokusatsu Tenshi Dokuro-chan the first monthly anime?
Yes

Up until the end of May 2020, Heyuri only recieved one or two posts a day on average (and most of those were just tomoposter tomoposting), and it was June 2020 was the first month of high activity on Heyuri (well, high activity on /b/ anyway). Towards the end of June someone came up with the idea to try and boost /a/'s activity by having us all watch an anime together

A thread was made asking for suggestions on what we should watch, and a while later a vote was held. At least one user complained (it was me XD) because the vote was missing some of the genuine suggestions people had made (including Dokuro-chan!), despite the vote also including Cardcaptor Sakura (which if you don't know, is 70 episodes long!! 0_0). The original vote, which had already amassed quite a few votes, was promptly considered null and void, and a new one with all the entries was created

The vote itself was riddled with many obvious fake/proxy votes, and it was pretty clear that Dokuro-chan hadn't actually won (and I say that as someone who voted for it!), but the staff went with the "winner" anyway because that's just how Heyuri rolls~ ( •̀ ω •́ )