You don't need a product.
You just connect the feed and vent to the pipe in an "H" configuration, the correct way around and the correct distance apart.
2 tees is all you really need... Use compression then you can always take it apart for future attention!
Discharge from unvented should point down to ground not out at the open world.
Does the kit in the photo have a NRV in the clamp-on fitting? Where's the NRV?
If it works as open vent, then add pipework with a fall on it... Enough to allow condensation (and rain ingress) to run back down to the stack.
And yes, use the proper drainage flexi.
Try it first and see what happens.
Only issue would be condensation if the flexi is fully supported, it should run back into the stack and not be an issue.
All depends on results from leaving it as a svp.
Another option, if there's room on the shower waste pipe, would be to access it via the ceiling below and fit an inline hepvo (waterless trap fanny trap).
In-floor horizontal 4" runs are not ideal but sometimes you've got no choice.
Is the whole stack internal or does it emerge from inside to outside at some point?
With the bag on the aav in the loft... Keep flushing the toilet and see if it eventually pulls the shower trap... Run the basin...