I’ve tried iPlayer on my Max and Motion+ and can get it extremely loud no problem.
In this example I found two places I could affect performance, one was in the Nebula connect app there’s a slider upper right move to right for more volume. In the screenshot it’s set to just under half volume.
The projector shows as I use the app the volume changes.
I paired my Motion+
I could alter volume in the app and on the speaker buttons
So in those two places I had the options and I could not take it to maximum as it’s too loud and I’d annoy someone in the house.
I also tried Netflix and there’s a volume button top right but I couldn’t find a way to affect it
So I don’t know what is going awry in your setup. I looked around Nebula settings for anything related to volume, the idea I have is I know sometimes in Android there’s an absolute volume setting but it’s not showing on this projector, I could probably nose around Developer Mode options to look but you’ve not enabled Developer Mode then yours should be behaving exactly as mine is i.e. there’s one volume on Max and one on speaker and between you should be able to get plenty loud enough as I can.
If you did happen to have the speaker paired at the same time with another device I know absolute volume can get in the way. If that’s the situation then toggle absolute volume to other than current setting. Simplest is to avoid dual-connection and turn speaker off / on between devices.