1. I agree, though here is a clarification: If you want to make custom GIF templates, simply press the Upload Video button and select the video file that you want to use, or copy the video URL and paste it. It is just like uploading a custom meme template.
2. I agree. For the meantime, if you know how to code, make a custom contrast theme with CSS.
3. The developers have ruled this out, as it would stray too far away from Imgflip's purpose.
4. There are many dyslexic-friendly fonts, including Imgflip's main fonts, or those on Google free fonts. If the font that you want is on neither of those, find one, download it, then add the font.