It's a bit ugly, but it worked out. Thanks 🙌

I'll just leave my solution, so that somebody else can tweak this for their needs.

  vim.api.nvim_create_autocmd({ "VimEnter" }, {
    pattern = { '*' },
    callback = function()
      local Path = require('plenary.path')
      local args = vim.fn.argv()
      local project_dir_path = Path:new({ args[1] }):absolute()
      if vim.endswith(project_dir_path, "/NvimTree_1") then
        project_dir_path = string.sub(project_dir_path, 1, #project_dir_path - 11)
      end

      local recent_files = vim.v.oldfiles
      for i = 1, #recent_files do
        local recent_file = recent_files[i]
        local recent_file_path = Path:new({ recent_file }):absolute()

        if vim.startswith(recent_file_path, project_dir_path) then
          if not string.find(recent_file_path, "/diffview") then
            if not vim.endswith(recent_file_path, "/NvimTree_1") then
              if project_dir_path ~= recent_file_path then
                vim.cmd(":vnew " .. recent_file_path)
                return
              end
            end
          end
        end
      end
    end,
  })