Hello and welcome @Rusthunter!
Rusthunter if/how I can setup my sandboxed Google account services into a separate user on my phone
Sure! You can have anything you want there. It's the closest thing to having many separate physical smartphones. Just create a new user profile and install the Play Store from the App Store.
Rusthunter can I still use the main user profile to access everything as normal without breaking my dependant apps (banking mainly) that had to be downloaded with PlayStore.
If you want to have an easy access to everything under Play Store profile, you can set it up as a Private Space in your main profile. You will find it in settings under Security & privacy.
Rusthunter The only g app I must use is Calendar
You might want to consider calendars from Tuta or Proton. From your other thread I see that you have to use it due to your job. You could try using it as a web app, further isolating it from your installed apps.