Good job on the coding side of things too. I'm not exactly an expert programmer myself, but it seems like you found some good solutions to the problems you were having with lag and what not there too. As you say yourself, they're all things you can learn from and reuse in future projects.
However, as for the game itself... I can see why it didn't quite catch on.
It's probably not the idea itself being terrible, because I feel the main game concept seems decent enough. Is it original? No of course not. It's been absolutely done to death a million times in the past, with tank fighting games being a long standing part of the medium.
But it's an idea than anyone can grasp really easily. So the potential for going viral was certainly there.
The issue was in how the game was presented and how it was marketed. For example, as much as your designer did their best, it just doesn't look very interesting as far as the art style goes. Really, it's the same old generic style I've seen in dozens of app store games nowadays. The kind that looks like it's trying to mimic the style of a Mac OS icon and not quite succeeding.
And that likely put people off. Remember, they've seen hundreds of games that look vaguely like this one. So they skipped your one thinking it just wasn't interesting enough to bother with.
So that's the first problem you had with the title. You picked a very generic looking art style, and used it with a game whose core designs had done a million times before. It just didn't stand out.
The second problem you had was marketing. Put simply, telling a few websites and setting up a social media account or two doesn't work nowadays. Seriously, unless your game is some amazing viral hit that blows up like Minecraft or Five Nights at Freddy's, you can't just rely on the whole 'build it and they'll come' philosophy.
No, what you needed to do was market the thing to YouTubers and other influencers. That's because those people have tens or hundreds of thousands of subscribers interested in the games they play, and usually in turn end up inspiring other people online to cover said games. Tons and tons of previously unknown indie projects have succeeded because they were lucky enough to have PewDiePie or the likes pick them up.
Hence you should have contacted these people (including some of the smaller channels) and tried to get them to do a video about your game. That's how a game tends to go viral now. Someone who's popular online plays it and says how great it is, and then their legions of fans and imitators give it a go afterwards.
That's just my opinion on the matter anyway. Feel you could have made the game look a bit more distinct and marketed it via YouTube influencers rather than just 'new IO game' listing websites.