Thanks for flagging, Connor! You are not alone with this request, others at Coda have run into this as well. There is not a work around just yet but I have added this to our features request list for everyone.
If you have any details you can share about your use case, we would love to hear them!
HI Maggie<
I have a very similar request, but for columns:
It would be great if the formulas explained above could be collapsed under “simple iteration”, it would make it much easier to get an overview of the page. (Especially once I add some “Complex Iterations”.
Below is a link to the doc
Regards
Piet
Here’s one workaround, You can put your content into Canvas columns and then @-reference those in your page. The formula will render the canvas (text and layout, and even buttons but no other controls.) You can then apply styling (incl. indent) onto the formula.
Please also add Lines, Images, Block quote, Code Block, Pull Quote, Formula,… and text that is higher in hierarchy that the collapsible text (example: h1 inside h3) to the list.
The inability to do this is the most disappointing part of Coda for me, as a user that came from Notion. As a “Docs and Sheets in one software”, it’s not there yet when talking about the doc part compared to other programs. But once the writing capabilities get on part with other doc editors, it becomes a no brainer to have everything here in Coda.
People like me see all the amazing and advanced parts of Coda and get excited, and then I want to write a very simple document and can’t put a picture under a collapsible text block… something so intuitive and something I never thought I wouldn’t be able to do.
I sense that the team is already working on this since it is a no brainer feature, but I want to express my disappointment… maybe it motivates the team 
Hi Fran! We love to hear feedback and I have added these to the collapsible list requests.
In the example you mentioned with collapsing an image, I do have a workaround for the time being! You can add a single space after the first collapse point, hit enter and have the image under the next collapsible level.
Another weird thing that happens when it tries to automatically place blocks inside collapsible blocks.
Hello! I was looking for the option to collapse code blocks and found this thread. After some more thinking, I realised that for simple code snippets, inline code formatting as described in this thread Inline code formatting works! Thought I’d post in case anyone else in the same boat comes across the thread. 
Would definitely still be handy to have collapsible code blocks for larger code snippets though.