Do you have any software related to window position and resizing, such like Magent, installed and running on your Mac?
I know this sounds ridiculous. But I got the same Macbook as you do. 2018 i9, 32G. And exactly same issue. From installation the NEXT button is unresponsive. When using UI button randomly freezing the whole application, and many icons just useless. The whole application just not in a usable stage.
Believe me I tried everything. Install different versions of Java, change memory size, non of them works. Until I quit the Magent, which I use to position and resizing windows. And suddenly TWS works like a charm, fast and smooth.
The same issue also happens on ThinkorSwim platform, which also use Java. I guess Java just don't play well with Magent.
That been said, if you don't have Magnet, here is my suggestion:
1. Download that ThinkorSwim, which also written in Java, and just see if that works. You even don't have to install it. Just run the install wizard, and if the "Next" button is not very responsive, you know it will have the same issue.
2. Quit all your background applications. Try run TWS in a clean Mac environment. And see if it works. Or maybe just reset the Mac to factory setting if that won't mean tons of trouble for you.
3. If TWS do work with a clean Mac, than you can start install your other backgournd apps back one by one, and see which will cause trouble.
So may or may not this work for you. Hope it will help.