Yes, you are right, the repeatable field group is based on custom posts, each item of repeatable field group is a single post, but Toolset form (CRED) can create/edit only one post in each form, it can not create the multiple posts in one Toolset form.
So Toolset form does not support the repeatable group directly, you will need to creating child posts in another form.
I am not sure when this feature will be available, and I don't think a method is coming soon.
For your original question:
I was thinking I could use generic inputs? And then pull them with PHP hook and put all in the body. So later it all just seems like regular html coded into the body like any other post.
There isn't such a built-in feature within Toolset form plugin, it requires custom PHP codes:
Here is my opinion:
1) You can setup a custom multiple lines field, for example "my-field", ask users to input the content using specific format, for example:
TITLE
TEXT
TITLE
TEXT
...
each item with title and text is separated by a blank line,
2) After user submit the form, use CRED action hook cred_save_data to trigger a PHP function, in this function get the value of field "my-field", and use PHP codes to turn it to a PHP array, for example:
https://stackoverflow.com/questions/3012788/how-to-check-if-a-line-is-blank-using-regex
I have searched it in google, and found some similar threads:
lien caché
For your reference.
3) Then use wordpress function update_post_meta() to update the field value to what you want.
https://codex.wordpress.org/Function_Reference/update_post_meta