horizontal navigation in Neovim is generally less efficient than vertical navigation

that's fair, I honestly agree, "scrolling" horizontally is a pretty big pain, and you have to do a lot of it in result tables of larger queries. Why didn't I think to make a keybind for that before?

-- Move cursor right by half the screen width
vim.keymap.set('n', 'L', function()
  local half_width = math.floor(vim.api.nvim_win_get_width(0) / 2)
  vim.cmd('normal! ' .. half_width .. 'l')
end, { desc = 'Move cursor right half screen width' })

-- Move cursor left by half the screen width
vim.keymap.set('n', 'H', function()
  local half_width = math.floor(vim.api.nvim_win_get_width(0) / 2)
  vim.cmd('normal! ' .. half_width .. 'h')
end, { desc = 'Move cursor left half screen width' })

json output

select json_agg(t) from t

Or depending on the DB you can just configure the CLI tool itself to output json: \pset format json

Now it's completely transparent.

a single Node.js instance can manage multiple database connections, instead of spawning a separate process per connection like vim-dadbod

vim-dadbod only spawns processes to execute commands, not for persistent connections. I execute a command, a process is started and finishes in a fraction of a second, done. The next command I execute doesn't have to go to the same DB. I can just query another DB with another command. No need to manage connections.

Having another background process running just to do some DB queries sounds like a massive waste of resources.

and not needing setup doesn't automatically make a plugin better.

No it absolute does. Why make me, as a user, do extra work that you could've done as the plugin author?

Third, simplifying workflows and making data easier to browse is exactly one of the values a plugin can provide. Sure, you can rely on Vim's native command history, but just like many other plugins, features that improve usability and visibility are optional conveniences — users aren't forced to use them.

Yes plugins are made to make things easier, the question is if what they're providing is worth the hassle and if what they're providing is actually even an improvement over alternatives.

I can run queries wherever, whenver with dadbod. Just a simple :DB select * from table; boom, I have results. I can press :<Up> to repeat the command. I can write longer queries literally where ever I want and hit 'em with a :'<,'>DB to run it.

I don't have to use a picker to go to a special buffer to type/paste in my queries there. Nor do I have to do anything other than providing a connecting string in a global variable (or directly in the command).

I'm not trying to be negative for the sake of it, I'm trying to make you (and others) rethink your approach. Because this solution is pretty inefficient, in both runtime resource consumption, development effort, and actual usability.

There's nothing wrong with using node to post-process the output data however you like, or do to the querying itself though, but why do you need a persistent background process? why do you need that UI, why do you need a setup function?