Ian,
I couldn't sleep, so figured I'd try to see what I could find for an aftermarket / upgraded trigger option..
No luck though, so unfortunately, I think you're stuck with the OEM plastic one. And you're not the only one..
There's a really nice wright-up on the H.A.M. site about your gun, and under the trigger section , the same thing was mentioned.
https://hardairmagazine.com/reviews/walther-parrus-air-rifle-test-review-22-caliber/ Years ago, I had a really nice crossbow pistol, and the maker went with a composite trigger..
Well, using it in freezing temps, I managed to snap it.. Luckily, I had all of the pieces..
I removed everything, super-glued the trigger back together, then used it to blueprint another..
Then proceeded to fab a new one out of metal. It is not hard to do - just a bit time consuming. But a fun little project.
All can be done using scrap metal or metal [plate steel] sold locally..
Trace the trigger onto some paper - make sure it's exact: All done w/ basic hand-tools too: cut that out,
transfer it to metal - mark/scribe it, cut it out, drill it out - if needed, then grind, shape, sand, paint/polish..
Over the years, I did a few more - simply because I didn't like certain triggers, and wanted to make some custom ones..
Just an option if you feel like doing a bit of tinkering..
Sam -