How to build single sitemap.xml that contains content grouped by type

What you are asking is possible. Have you read the documentation?

SITEMAP VARIANTS
It is possible to have several sitemap instances of different sitemap types with
specific links accessible under certain URLs. These sitemap variants can be
configured under admin/config/search/simplesitemap. The module comes with the
default sitemap 'default' which is accessible under /sitemap.xml by default.
There is also the 'sitemap index' variant which is disabled by default and which
can be used to index all other variants. If there are multiple sitemap variants
it may make sense to enable the sitemap index variant and make it available
under /sitemap.xml by setting it as the default variant in the module's
settings.

Let me know if this helps.

On a side note, I know I mentioned it a few times to you, but it makes sense to create support requests if unsure if a bug or feature exists. Also when creating bug or feature tickets, it doesn't make sense to tag a stable version of the module. I can't go back and add a feature to a past release - we only work on the dev version.

Thank you for the prompt response.

Part of the problem is terminology - "sitemap" vs "sitemap variant" vs "sitemap type", until you become familiar with what each term means in practical terms it's confusing to read the documentation. It's kind of like the "blocks" system in core - the same word is used for four or five different things, it's confusing until you gain more familiarity with the system.

After fiddling with some settings I was able to achieve what I wanted, thank you for the pointer.

I did work on some documentation to explain how to achieve the scenario I was after, along with a minor improvement to the SettingsForm. I think further changes could be done to make it the distinction more clear on what a "sitemap" means in any given scenario, again similar to the word "block" in core.

I'll open a separate issue about changing the URLs, as I think there might be a bug there.

And yes, I'll keep in mind that my requests have turned into support requests and will start there in future.

Again, thank you for your time, I appreciate you.

I'm closing this support request as fixed. If you don't mind, please create a new task issue with the documentation tag and create an MR instead of a patch. This massively helps reviewing and commiting.

Automatically closed - issue fixed for 2 weeks with no activity.