I am not super familiar with latex am currently writing my bachelor's thesis and kinda close to despair.
I need some really big underbraces for one of my proofs and the don't render properly and look super wonky. I am using a template from my uni with \documentclass{scrreprt} and \usepackage{amssymb,amsmath,amsthm,mathabx,bm,mathtools} imported, I don't use or change fonts anywhere. The template previously had \usepackage{libertine} and \usepackage{libertinust1math} imported but I had to get rid of that because it made some letters look odd and my supervisor wasn't happy.
I figure changing the font type of just underbraces to \libertinus1math should work? I can't get that to work and under fixes for the wonky underbraces that I found on similar threads just produced error messages so far... The only fix that has kind worked was desplaying the underbraces in text style (sorry, can't find the post anywhere) but that made them really flat and wide and they woudn't fit in my margin anymore.
Please help, I just want to get back to my proofs and not spend ages on type setting issues :/ I added some pictures, I hope this helps in assessing how to fix this issue. 

mathabxhelp? That loads some really old math fonts and has a number of issues. Anything you need from it is available elsewhere. – Davislor Jan 07 '21 at 08:23