My current method is by using register_block_style(). I believe it's future-proof, but I hate seeing a lot of options in the WP Button Style tab. Also, I do not like registring new style just for the icon. For example, the client asks to add a Download icon. I have to edit the theme code, ask the client to test on staging, and deploy to production. It's a small addition, but takes a lot of effort to do so.

I am wondering if you guys have any method for adding icons in WP Buttons block you can share.