I was just looking for the same thing. I found web_groupby_expand but that only goes up to version 13 (and will break your server if you install it on 14+). After a bit more digging I found that there is a similar, but maintained, module called web_group_expand in the OCA repo https://github.com/OCA/web/tree/14.0/web_group_expand

Note that it's for version 14, but that's better than trying the one for 13 (which is for sure going to break your site).

Basically what this module does is it adds 2 buttons in tree views, one to expand all and one to collapse all.

By "all" it actually expands the next groupation, so if you have 3 nested groupations you're gonna have to click 3 times but that's much better as I found no alternative.