Only found them by coincidence ;)
1️⃣ In "/settings/preferences/appearance" under "Advanced settings" you can find the option to "Warn me before posting media without alt text"
You can see the error message I see whenever I forget alttext in the second image hehe
2️⃣ When you post an image with text in it, you can simply use the magic AI feature "Add text from image". It usually works very well!

Alt text: "Screenshot of the warning you get with the warning for missing alt enabled. "Add alt text? Your post contains media without alt text. Adding descriptions helps make your content accessible to more people." Below it are 3 buttons "Cancel" "Post anyway" and "Add alt text"

Alt text: "Screenshot of the alt text window, the option "Add text from image" is circled in red.
This is how that "AI" option described the text of the image I posted: "Describe this for people with visual impairments. 1500 Add text from image"