The warnings you’re seeing come from this change to which a PR has been put up for user-setup but not merged yet.
Could you provide a snippet from your ~/.vimrc that includes how opam_available_tools is assigned a value?
PS. This might be overkill for what you are trying to achieve but a member of the community put together a good write-up on his vim setup here: NeoVim setup in Lua for OCaml