Badges don’t get much love in standard Lovelace, and card_mod is probably your best option. See here for how to style badges. It doesn’t require custom_ui, which is no longer maintained.
Agreed that it would be nice to have more flexible badge support. I, and many others, just moved to panel mode and built “badges” with a set of button cards or something similar. Much more flexible, but also a bit more work to get going.