Thanks for the feedback. I attach a squashed updated patch for part 2.
You can also see the unsquashed changes at https://github.com/jackkamm/org-mode/tree/2024-grouped-weektree-rebase

>> +(defun org-datetree-find-create-entry
> Please also document how `org-datetree-add-timestamp' affects this function.

Done.  On reviewing this I also found a bug (datestamp added again if
entry already existed) -- I fixed it and added a unit test.

>> +     ;; Support the old way of tree placement, using a property
>> +     (cond
>> +      ((seq-set-equal-p time-grouping '(year month day))
>> +       "DATE_TREE")
>> +      ((seq-set-equal-p time-grouping '(year month))
>> +       "DATE_TREE")
>> +      ((seq-set-equal-p time-grouping '(year week day))
>> +       "WEEK_TREE")))
>
> It would be a good idea to add a few tests for this scenario.
> To make sure that refactoring did not break things.

There was already a couple tests for DATE_TREE and WEEK_TREE, but I've
added a few more now, mainly around finding existing headings (the
previous tests only created new headings under the DATE_TREE or
WEEK_TREE).

> Why do you need object granularity by default (second call to
> `org-element-parse-buffer')?
> Also, more importantly, do you have to run the full parsing here? Maybe
> utilize `org-element-cache-map' instead? Full parsing is going to be
> much slower.

I've switched to `org-element-cache-map' now -- thanks for the info
about it.

> It is undocumented in the `org-datetree--find-create-subheading'
> docstring that it returns something.

I changed the return behavior and documented it.  It now returns non-nil
if the subheading already exists -- this is needed to prevent
adding a datestamp twice when `org-datetree-add-timestamp'.

Finally, I made one more substantial change -- I now allow :tree-type to
be a function in the org-capture template.  This allows using new types
of datetrees from `org-datetree-find-create-hierarchy' with org-capture.
Relatedly, I made `org-datetree-comparefun-from-regex' public to help
with building new types of datetrees.