When I was learning neovim over the summer, editing the configs was the perfect way to practice editing. Lua is a simple language and forcing myself to do the config from within nvim created a positive feedback loop where I wanted to keep using the editor more and more. (I realize this anecdote is a non-solution to your request)
I think lazy.nvim has a pretty decent UI box for when you have plugins already installed. Things like managing updates and uninstalling for a session. But yeah it would be such a mess for a separate program to mess with the actual nvim config, or it would require enough setup to make it not even worth it in the first place. 99% of plugins can be “installed” by just pasting their remote Git repo urls into your package manager setup so, it’s not bad really. Part of the trade off for more control and efficiency I guess!