Q: Multiple font changes without repeatedly opening menu

You can use the key board equivalent as below. "Command" and "U" for example, to change to or to cancel underlining.

You're welcome. It occurred to me I didn't directly answer your question... No, you can not have an active menu open while you work or type elsewhere on the computer. But, you could take a partial screen shot like I attached to my previous reply and leave it open on the compute alongside what it is you are woking on like example below. Also, there are tons of Mac keyboard shortcuts you see at this link (Mac keyboard shortcuts - Apple Support). I only remember and use a handful of them, but they come in very handy at times.