I am planning on using Umbraco with content delivery API as backend and build a SPA client frontend. My plan is to use the same base URL for both so that example.com/umbraco may be reached by editors and visitors go to example.com/ But what about routing? Will my client routing allow /umbraco to break out and maybe more importantly will Umbraco let example.com/anyslug be reached by my client? Any thoughts / experiences on this? Is it better to just use different addresses / ports?

The routing may be an issue as you say. Someone else might be able to tell you for sure. You could give it a try and see what you find, but I would be tempted to put your front end SPA on one URL (e.g. example.com) and your backoffice on another (e.g. api.example.com, or api-example.com), then use an API controller maybe to return the JSON to your SPA?

Found this that I am reading now. Sounds quite similar to what I want to do. But perhaps a separation might be good anyway. https://dev.to/mozaky/series/22504

I'd suggest having them seperate to avoid issues... Thats what I've done when I've made SPA's atleast... But I couldn't tell you exactly how much work it would be to have them run on the same domain.

+1 for hosting on different URLs. You can usually configure a SPA router to ignore a path, but maintaining that long-term is awkward, with not too many benefits