The decrease of different #1s in a year is because of 2 reasons:
1. The drop of downloads and rise of streaming. Downloads obviously just reflect its impact in the short run unlike streaming. Prior to 2012 the digital market was completely dominated by downloads, which translated in songs having a super short run in the charts. In 2013 there was a price change in downloads which made them drop, streaming started to rise, but the digital market was still in the rise, so downloads were still really relevant and even rose after the initial drop of 2013, repeaking around 2017-2018. In late 2019 it started to be noticeable the download market was decreasing a lot (Gaon/Circle hide it but we could see it through the certifications), streaming started dominating everything to the point downloads are almost irrelevant nowadays.
2. The system change of the charts in 2020. Summarized, they changed realtime charts with 24h charts, making it way more difficult for new songs to chart high. For some reason koreans seem to focus their streaming habits on charts a lot, so this caused a lot of artists that were guaranteed hitmakers thanks to people noticing about their songs through the charts directly disappear. Now promotions are way more important, that's why only we see few non idol artists topping the charts nowadays.
I didn't expect this to be this long, sorry lmao.
Regarding the topic, honestly the charts and the streaming habits of Korea change that much that I don't think we can measure something like competition with numbers. Also, Kpop dominates the charts now since 2022 I'd say, and it did during the 2nd gen (when Kpop probably peaked in Korea), but the truth is that during the 3rd gen most of the biggest hits of Korea were coming from soloists and non idols. Half or maybe more of those #1s during 2015-2020 are probably not idols, with some cases like 2019 when we didn't have any group song topping the charts until Psycho did in December. The korean charts are a representation of the korean public, not of Kpop.
After all this boring essay... IMO the most competitive year was 2017, like I said I don't think we can measure it just with numbers, it's just my feeling as someone who has been checking the charts for longer and more frequently than I'd like to recognize lmao.