Suggestions:
Standardize things. Don't use a bunch of different fonts or typefaces. Use the same font. Standardize the text size. Standardize the background colors. If you want to differentiate between things like speech and thought or narration, maybe try making minor adjustments, like italicization of text, or the style of bubble (e.g. rounded bubble vs squared bubble), but whatever you do, keep it simple and limit your variation lest it becomes too confusing.
Don't use a typeface or font that is too stylized. Try something minimalist and simple. Also, I'd probably try to avoid using bold by default, especially if the typeface is already rather thicc.
Color choices. Don't use solid black or white as a background for your text bubble. Use more neutral colors, like charcoal greys or off whites. Your text colors will depend on your background color choices, but generally I wouldn't suggest super saturated text color.
Your color choices should stand out from your render colors, too. For example, in the BlueTalk attachment image your text color is very close to the dominant color of the render, which is the alien woman's skin color. IMO, you should avoid doing that.
Also, think about composition, or to put it a bit simply, the way our eyes are drawn across the image before us. Again, using the BlueTalk attachment image, your text is at the same height in the image as the woman's face, and is right beside her face, so the eyes are naturally drawn more to her face even while reading the text. Good composition tends to direct the eyes of the audience naturally across the whole image, usually in a triangular or circular motion. The RedTalk attachment image has a more comfortable composition and ease than the BlueTalk.
I'd also suggest depth of field effects for your images and renders if you're going to put dialogue and text into the renders like this. Noisy backgrounds will make things harder to focus on, but using some depth of field blur will probably help to alleviate some of that, and better control your audience's eye path and attention.
Idea: Maybe instead of just using a solid colored background for your text, you use a background that is just heavily gaussian blurred.