Not only is UPER hard to parse, but (I believe) 3GPP ASN1 definitions are provided only in .docx files which aren’t exactly the easiest to work with. It’s just really not a fun domain.

The ASN.1 format itself isn't too bad. It shows its age and has some very weird decisions behind it as places, but it's not that difficult to encode and is quite efficient.

Unfortunately, the protocols themselves can be confusing, badly (or worse: inconsistently) documented, and the binary formats often lack sensible backwards compatibility (or, even worse: optional backwards compatibility). Definitions are spread across different protocols (and versions thereof) and vendors within the space like to make their own custom protocols that are like the official standardised protocols, but slightly different in infuriating ways.

If you parser works (something open source rarely cares about so good luck finding one for your platform), the definitions extracted from those DOCX files are probably the least of your challenges.

First you can download specifications in either PDF or doc(x). Second doc(x) are simple enough that simple doc(x) to ASCII/text is good enough to produce working ASN.1 definition. Copy&paste is also an option.