I have the exact same issue. After the recent updates, I have switched over to Dynamic Fonts. Where is this “Custom” option you are referring to David? At the bottom of Typography, I see “Other > Configure.” But it won’t let me put a value in any of the target elements.
OK, so this is what I see on my end on all my sites. Really weird. Same on two different computers. And I’m not zoomed in. https://jmp.sh/Hn2FZdu It won’t let me scroll all the way down because the dropdown goes off of the screen.
The good news is I’m able to type “Custom” and that seems to get me there still. Also if I move my mouse and scroll instead of trying to drag the scrollbar, that also seems to work.