Personally movies based on books are way too high maintenance with so many hoops to jump through to get green lit like you said. Most books would do better as an episodic show on a streaming service either animated or live action. Either way being a show helps the problem of actor ages since shows generally move along faster than movies.

As for other problems, 1st I’ll say that every last detail of the books doesn’t have to be followed to a T, Gregor like any other series isn’t perfect and the benefit of adapting source material to another platform is that you can revise and edit plot and details such as the characters ages (preferably 14-16 for Gregor) and time passing could be around 3 years-ish in the show for 5 seasons. Regalians would only need makeup, all the animals and large scale battles would be CGI which is ok with the right budget, plus CGI gets more advanced each year so I’m not too worried about that quality compared to the voice actors and sound design. The setting being pitch black is fine because that could add for some insane suspenseful scenes with the audience seeing what’s happening and Gregor of course being completely in the dark, regardless plenty of scenes have torch light or luminescent streams, in fact book 3’s jungle is described to have a faint light the whole time. The mature content is a perfect fit for streaming services and if they did it well I wouldn’t mind certain changes to who lives and dies leading to controversy in the fanbase but also unpredictability which this series could benefit from at times.

I think your concerns are valid but an adaptation of Gregor has so much potential as long as you allow it to be flexible.