I think it was in Ubuntu MATE that I saw a dialog, probably in MATE Tweak, to choose from a number of different look-and-feels. I much prefer the single menu instead of Applications-Places-System. Does Devuan MATE have an option to do so? I didn't see it in MATE Tweak.

Yes, you can remove all that from the panel, and add the menu of your choice. I'm not in MATE now, but if you right-click on the App/Place/Sys thing, you can delete it. Then right click on the panel and add the menu of your choice. Some people like the Brisk Menu (I don't). There's also a simple menu. Just poke around and see what you like.

No, this is not something you do from MATE Tweak.