Alt-text descriptions should clearly convey both the content and the meaning of the image, and should aim to use as few words as needed. Describe what's essential to understanding (and enjoying!) the intent of the posted photo — you don't need to add in a sentence for every visual element, but should include as much as you need to create an accurate portrayal of the image. Cut out unnecessary words and combine separate sentences as much as possible. One to two sentences is usually more than enough room to describe what's going on.
As mentioned before, these photos convey information to the people scrolling your page, even if you are just posting them to brighten up your feed. They have a purpose, and for that reason, alt text should focus more on the image's meaning than its aesthetics. This means you're not focused only on what the object in the photo looks like, but what it is and why it was posted.
I was hoping to see a format that people can easily follow and just fill in the blanks, but I suppose this is the gist of it: Describe the main purpose of the photo succinctly rather than each and every individual thing you can see.
Every damn time. IMO, it's not really free if it requires a payment method; free trials should automatically end when the time is up instead of making you the one responsible for canceling to avoid being charged.