I just make them from scratch with FireAlpaca as I also animate them frame by frame. I start with a transparent 88x31 canvas and create two layers, one for the background and one for the border. Lately I've found it more useful to design the border first as it sets the tone for the rest of the piece. Some fancier border designs will also require more space; so it's good to what you are left with before you start the main content rather than having to make sacrifices later down the line.
Readability is definitely a big concern. I played around with a bunch of different fonts and found that Century Gothic is very readable at 3pt, so I've adopted it for pretty much all of my buttons. It is a little samey, so I'd love to hear if anyone has any other font recommendations.
I've definitely had to abandon button ideas since the assets I was using didn't scale well enough; sometimes I can cheat it a bit by imagining the button to be like a window or veiwport; I keep the assets at a large scale but animate them so most of the image crosses into view, but this is not always a viable solution. In these cases, the idea might be better off retooled as banner ad rather than scrapping it entirely. I'm not sure what the agreed specs are on that but my (horizontal) banner ads are 468x60.