Saundersfoot bay is a great location for sibbing, and the harbour has an easy slip, but a couple of things to watch out for:
1. The 2.5 hours either side of high tide seems a little optimistic to me, even with a SIB. The 'low tide' photo above is probably nearer to half tide - at low tide the harbour dries out except for a stream through it. There is a soft sand / mud bank between the slip and the main channel which could be a limiting factor in getting in / out as the tide falls (hopefully someone with more direct experience can comment).
2. It looks like they close the slip outside of working hours now - when I was there earlier this year there were locking bollards across the slip. Not an issue if you are deflating the SIB anyway (or can tilt the SIB over and carry between them), but something to be aware of.
There is a pedestrian ramp on to Saundersfoot main beach (just to the right of the 'low tide' photo). With a light SIB you could park near the top of the ramp and carry down onto the beach and inflate. Depending on tide state it could be a long walk to the sea, but if the SIB is light enough for the number of people and you have decent transom wheels it is doable.
As a final option, Glen beach is a little way up the road - you will need to carry the boat down to the beach on its side - when I was a nipper we used to do it with a SIB and 4.5HP outboard so again with enough muscle it is doable.
Best suggestion for all the above would be to recce the area at low tide first to avoid getting caught out, or aim to launch and recover on a rising tide.
As an alternative, Milford Haven waterway is great for sibbing. You could launch for free at Black Tar (IIRC free slip and free car park):
Blacktar Haverfordwest[slipway] - Boatlaunch
or East Llanion (also free):
East Llanion Pembroke[slipway] - Boatlaunch
Plenty of other slips but those are the two I know.
I would stay above the Cleddau Bridge, and stick to the main rivers until you have gained experience (and have a good chart!).
Information on the Milford Haven waterway here: