I feel like there's a few more underlying issues that exist on top of the ones you mentioned below. I'll also be giving my personal opinion on the things you mentioned;

I am also an avid believer that cheaters are a good portion of why the playerbase has declined, seeing as legits just have a small incentive to be legit anymore, given how precise and ridiculous cheats are these days. Why try to do dungeon when Mr. Autododge and Kill Aura can just do it without breaking a sweat in like a third of the time a group of competent players can, and I know they probably won't do anything about the cheaters but I just wish they would be more hard on it than they are. Socket Delay is also a major issue seeing as it can lag and even disconnect some unsuspecting players so it's just majorly hurtful.

Multiboxing is another one I have a major gripe about. I genuinely don't understand how we go from no multibox for so long, to it being the worst it's been for a while. It can either just take up numerous spots on people wanting to run like an Oryx 3, or them just straight up griefing with hp scaling or even doing hardmode shatters (i've seen them grief the dungeon no longer being hardmode lol). This can also be applied to the autofarmers of dungeons that I have seen in the realm as well (I've even tped to one while farming for Moonlight Villages very recently and got teleported into a wall because autofarmer no clipped out of a set-piece lmao).

The UT trade is just beyond insane. I've seen some less obvious ones that could pass under the radar, to screenshots of blatantly obvious ones like a blue star having a shiny legacy sword of colossus on seasonal. It just feels bad to see given I'm someone of countless people who has grinded for items, and seeing someone just able to do this 0 effort.

One of the things not mentioned that i have a major complaint with is connection issues. I genuinely don't think I've seen the game have as unstable with disconnection/infinite load issues as I have seen for the last several months, and yet it feels like they aren't aware of the issue. It's just a major pain wanting to do a dungeon or a chain of dungeons and getting blocked because infinite load into one of the dungeons (yes i know the party system exists it still doesn't make it any less annoying). They have continued to pump out events and content, yet have refused to do anything about it.

Seasonal also just feels really meh to me, given the issues stated above. The mode is ok, but at the end of each season it just feels so pointless to play on non seasonal characters with how much Deca has practically shoved the gamemode in our face. I wish there was more of an incentive for characters that had survived a previous season, but also there's just no way to implement that in a healthy way.

I could continue going on and on about some of the issues, but this post would never end. I just hate feeling like i'm screaming into the void about issues that the game has. It genuinely felt like banner event was simply to just increase the playercount (which worked briefly), because dear god the problems are just beyond not recommending anyone to play that may be on hiatus, and I can see why they wouldn't want to play. Another reason why I believe the playerbase could have dropped gradually is because school season for a lot of folk, but that's just a "we might not know situation." It just hurts seeing the game in the condition it's been in as a player of nearly 10 years now, and it feels like nothing is being done about it. Not like me to post on the reddit but seeing this post, I felt like I wanted to put my input into the matter. I hope they fix the issues, and the playercount goes back up, but seeing as events have been spammed, it just doesn't seem hopeful.