For Spotify, you need to use their share → embed track → copy feature and indeed add https://open.spotify.com/ in your allowed iframes.
Tiktok and IG reels share codes use a blockquote along a <script> elements, so it won’t work straightforwardly as script tags aren’t allowed in posts.
I also suppose Discourse’s onebox doesn’t support them yet.
I don’t know if there are any working iframe links that could display them.
Did you use the search? I quicky found those:
Tiktok embedding is discussed here:
You can embed tiktok video like this:
For example, for https://www.tiktok.com/@mobsao/video/7181493818871565574
<iframe src="https://www.tiktok.com/embed/v2/7181493818871565574" height="800" width="400">
Instagram reels embedding is discussed here: