Quality is good on the Reimo Easyfit (providing the installer does it properly), I have a SWB with a basic foam mattress and solid bed board. You mention keeping the bedding in place, I’m pretty sure that isn’t possible on the Reimo. The basic mattress is thin (approx 20mm) and not that comfortable, I prefer to sleep down below.
I also queried the belt down securing but this is a pretty safe method, I’ve seen camlock and similar latches fail on boats and if that happens on a pop top roof it’s going to be disastrous.
Headroom is fine, we are both around 1.8m and no issues.