quesada welcome to the forum!
Since your post seems unrelated to the Rich Embeds extension I have split it to a new discussion.
There are a few existing discussions about using iframes in posts already, but I think none of the discussion really went anywhere. The gist is that making a general-purpose iframe extension would likely bring more security issues than any benefit.
Flarum has a feature called MediaEmbed (which can be enabled with this extension) that allows embedding a list of websites into posts. The logic for embedding those websites is pre-defined and therefore safe. There's a list of supported website here https://s9etextformatter.readthedocs.io/Plugins/MediaEmbed/Sites/ but I don't see "amplenote" in the list.
The best solution would be to create a small extensions that registers a new MediaEmbed rule or a custom bbcode that converts a pre-defined URL into an iframe using the Flarum TextFormatter API. This is the safest way to make external content embeddable in posts by regular users.
For images, Flarum doesn't come with any built-in upload extension, but there are a few options available from the community. We didn't install any on this particular forum, but you can hot link images from external hosts (like Imgur for example).
