The longer game is to change where they live, because you are not fighting the slugs in your bed, you are fighting the ones in the metre around it.
What helps over a season or two:
Remove the daytime shelter next to your beds. Slugs spend the day under things, planks, pots, dense weeds at the edge, long grass against a bed, an old bag of compost. Clearing the strip immediately around a bed does more than anything you put on the soil.
Water in the morning, not the evening. A bed watered at dusk is damp all night, which is when they feed. This is a free change and it is worth more than most of the remedies.
Encourage the things that eat them. Ground beetles, frogs, hedgehogs, birds. A small pond is the single most effective thing if you can have one; a log pile and a gap under the fence are the cheap versions.
Accept a sacrificial zone. A few plants at the edge that you do not mind losing take pressure off the rest.
And the reassurance: a night like yours is usually the worst it gets, because it combined freshly planted, freshly watered, soft seedlings on a warm damp night. Change two of those four variables and you will not see a repeat of that scale.