I was having similar issues and it sounds like we found a lot of the same posts when searching for help. I did manage to fix my issues by installing VS 2019 Build Tools: https://visualstudio.microsoft.com/thank-you-downloading-visual-studio/?sku=BuildTools&rel=16
This is the thread I followed which got me to the fix. filipw is the person who posted the fix if you want to read through it: https://github.com/OmniSharp/omnisharp-vscode/issues/4340
Once I installed that and rebooted, the autocomplete/intellisense started working for me. Hope this helps.