I’ve installed dozens of acrylic showers and shower over baths . I have never used any bracing because of the amount of adhesive and how it’s applied when done properly, bracing is not needed. And what you have is completely pointless. If you can feel air gaps then I’m suggesting installer hasn’t applied enough adhesive and definitely has smooth out the liner hard enough against the gib . Another rookie mistake is to take to long to glue up and then put liner in - the adhesive skins up quickly making it difficult , if not impossible to smooth out . I would have 4/5 tubes cut and open for your job . Once I start applying I don’t stop going top to bottom as fast as I can . Lines about 70 mm apart and then throw the liner in. Then I get in with a couple of towels and smooth the liner out against the wall . It’s a real work out and literally pressing every part of the liner HARD down . Your pictures suggest this install was done by someone who doesn’t know what they’re doing. Gaps + liners+ water can be problematic. Best you have a conversation about it with the guy More on reddit.com

Designing Kitchens and Bathrooms - Wallspan PVC panels (by Dumaplast) for shower stall - Our home improvement store has Wallspan in the tub/shower area. We want to remodel our shower so that the entire wall and ceiling are covered. All the shower surrounds (except for one-piece units - which ... More on doityourself.com

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I’ve used them, and like them a lot. I agree with everything you pointed out. One thing you didn’t mention was how easy they are to keep clean. Soap and minerals have a hard time sticking to the walls. All you need for cleaning is warm water and a sponge. With no grout lines, mold and mildew build up in the crevices is reduced or eliminated. The downside is that it looks like a motel shower. And that is off-putting to some folks. You are not going to customize this in any way. So it is a generic shower. More on reddit.com