I too know these locations well and would agree with Phil... Aldeburgh needs treating with real care. It's not at all ideal for a sea newbie in a low powered SIB with very young family. In truth "20ft out" can be much more challenging than 2000ft out! Those of us that beach launch want to spend as little time as possible in that 20ft zone!
Launching then recovering from a steep shingle beach into breaking water (even if it looks minimal from a paddling point of view) could spoil the whole experience.
Southwold a little better with its more gentle beach but the harbour entrance can be very very dangerous.
Do you need to go to that area... I knows it's really nice as I've been going there on and off since a child... 50+yrs now... but there are other much better places for you.
We live in Cambs just up the A1 from your area and our place of choice for our 3.4m SIB is West Mersea. It is about 10mls nearer then Aldeburgh from you and massively more suitable for getting eased into the sea.
Have a look at my report with images on page 2 of this thread...
Launching question West Mersea
Totally perfect for you with the safely sloping slip away from swell or breaking water, pontoon for taking a break, cafes, toilets and a choice of bravely motoring out towards sea a little through the moored craft (always something to paddle to and tie on if the engine fails!) or staying in the creeks to build your confidence.
BTW for what you want to do it would be the Mariner for me every time.
Anyway whatever you do I hope it goes well... and stay safe!