It’s been 3 years that I am using Neovim as my daily driver. I learned a lot of things in the mean time. The only things left are macros and registers, which I don’t really use, except registers for basic stuff. Here is a list of things that come to my mind when I think about Vim (not the modal editor, rather the builtins):

• Be lazy when you load plugins: I’ve been using packer for two years and it was all good, I could get a startup time below 50 ms with a dozen of packages. Now that I use lazy, I got comparable improvements in startup time with even more packages.
• Don’t forget builtin commands: There are a lot of useful commands already available in Vim, and many shortcuts that are worth learning, e.g., for manipulating windows and tabs or the z and g prefixed commands.
• Search and replace using regex are powerful tools: Use capture groups eagerly, like in :%s/\([0-9]*\).*/\1/; also, don’t forget you can save search results in a file, e.g. g/pattern/.w! >> out.txt, if you need to process them afterwards.
• Prefer a low contrast monochromatic colorscheme to avoid Christmas tree-like effects; treesitter should take care of highlighting the right stuff, and it will be helpful to perform structural editing.

• Use templates for your one-liner: You can define a set of autocommands or a simple mapping to insert a file right into your buffer; in the latter case, :-1 read allows to place a template file above the insertion point, in contrast to the default behavior of :read.
• formatprg and equalprog are invaluable if you don’t use or don’t have a relevant LSP at hand. Vim is quite good at indenting, so you’re more likely to deal with formatprg. For instance, to format bibliographic entries in a Bibtex file, you could define setlocal formatprg=bibclean\ -align-equals\ -brace-protect\ -delete-empty-values\ -fix-accents\ -quiet in your after/ftplugin/bib.vim, and then gqip the current entry.
• You usually want to put your custom settings in after/ftplugin rather than plugin. If possible, keep it orthogonal to your init file so that you can use a minimal init file that is shareable across machines while your after folder get you load of enhancements to the core config.
• Read the documentation (and learn to navigate inside) rather than spending hours browsing the internet to learn about the latest cool plugins (which is only compatible with the next-to-last version of Neovim); that is, write small functions or commands that do 90% of what you expect, rather than plugging in yet another plugin. Most of what you will read from past Vim wikis still apply as of today in Neovim, so don’t reinvent the wheel.
• We rarely need to <C-z> these days, thanks to the builtin terminal emulator.